A few well-meaning answers here that are right, but for the wrong reason, so going to set out the correct process for others who may come across this.
Rules for your ticket: https://www.nationalrail.co.uk/Bd
Your ticket’s not valid to use a train that arrives in London before 2pm. The 08:35 arrives in London at 10:49, so that’s not valid for use.
If you do use that train, you won’t be charged a full new ticket, however – in line with National Rail Conditions of Travel section 9.5.
9.5 Where you:
9.5.1 are using a time-restricted Ticket (such as an “off-peak” or “super-off-peak” Ticket) that is correctly dated but invalid for the service on which you are travelling; … you will be charged the difference between the fare that you have paid and the lowest price Ticket that is valid for the train you are using.
That would be £109.20 less the £46 already paid, or £63.20. In practice, you would probably be sold a new single fare for £54.60.
Now, what you can do, which is a little off-piste, is take the 07:56 from Sherborne. This train terminates at Basingstoke and does not go on to London, so does not fall foul of the ticket restriction. You would then get off at Salisbury at 08:37 and take the 08:50 to Romsey*, again the train doesn’t go on to London and isn’t restricted. Alight at Southampton Central at 09:28, then it’s an hour wait until the Brighton train at 10:28, arriving 12:21 like you wanted to.
None of the trains you are using go to or from London, so the time restrictions don’t bite.
A health warning – this is a validity loophole. The majority of super off-peak tickets on South Western don’t become valid until 11am. It’s pretty much a given that the train conductors, if you encounter them, will challenge the validity of your ticket pretty heavily, and even though it is valid for the reasons I’ve explained, you may well have to argue and explain yourself. You may even be charged an excess fare (though you won’t be "fined" as another posted incorrectly suggests; only courts and police can issue fines), and that may be difficult to get back.
If you want to take the low-risk route (or if 07:56 is too early for your liking), you should cancel your ticket and rebook for the desired journey. There’s currently no admin fee to cancel a super off-peak ticket assuming it’s submitted for refund by 18:00 on the day before travel. If you can’t get an advance fare, you will be best buying two single tickets, one there and one back.
*The Romsey train calls at Romsey twice in a "figure of 6" arrangement via Southampton, so may be identified with the destination of Chandlers Ford on the departure screens at Salisbury.
As others have mentioned, your ticket is not valid on the earlier train. Edit: island explains the actual procedure for travelling with an inapplicable time-restricted ticket in his/her excellent answer elsewhere. That said, (s)he also notes that the guard may need to be gently walked through your arguments in respect of a cunning routing plan, and in my experience, this often applies to procedures like the one detailed in s9.5 also. Which is to say that you may end up having to pay a large penalty fare on the train, then applying for it to be (mostly) refunded later.
So I thought to discuss what you should do about it. As I understand it, your ticket is not an "Advance Purchase" ticket, it is merely an ordinary open ticket bought in advance. As such, refunds are governed by the National Rail Conditions of Travel. s29.1 specifies that
If you purchase any Ticket(s) and then choose not to travel, you may apply for a refund
from the original retailer, unless the terms and conditions of your Ticket(s) state
otherwise. You must do this no more than 28 days after the expiry of the Ticket(s)
unless the terms and conditions of your Ticket state otherwise.
Although s29.2 permits the retailer to keep a small admin fee (up to £10 per ticket) I’ve never experienced this, and have got prompt and complete refunds when I’ve needed them, although I would also never buy an open ticket in advance. That said, in these straitened times, it can only be a short time before some bright retailer realises they can hang on to even more of your money.
So I would go back to the website where you bought the tickets, see if it has a facility for applying for a refund (and hopefully no special T&Cs that bar this), and ask for one. Then use such a refund as you get to buy an Off-Peak Return, which seems to be the ticket that covers the journey you want to make (assuming an acceptable return time). I confess that I think the price difference is £26.30 (£106.30-£80), though, which makes me wonder if I’ve failed to grasp something about your case.
I was wondering if it would be acceptable for me to just get into the earlier train without booking the ticket
No
or would I get into trouble for it as it’s a different peak time?
Yes. Your off-peak ticket is not valid on a peak train, so you are riding without a ticket. If you get caught, at the very least you will have to pay the full same-day price for the train you are sitting on. The conductor may also choose to hit you with fine for riding without a valid ticket but that depends on how they typically handle this case.
Should I be worried?
If you are riding a train which you don’t have a ticket for you should indeed be worried
I ordered these tickets online and the price difference around £13, I’m more than willing to pay the difference but would like to know if I needed too?
The ticket came with terms and conditions that you agreed to when you bought it. These terms include the change rules. Maybe you can change the ticket for free, maybe you have to pay a change fee, maybe you can’t change it all.
Look, at 8:35 the trains are packed with commuters, at 12:35 the trains will be a lot emptier. That’s the reason why the train operator offers off-peak tickets for a lower price: to incentivise people to ride at 12:35 not 8:35. You can not expect to take advantage of this pricing and still be able ride the peak train . You need to buy a ticket that’s valid for the train you actually want to ride.
I don’t think this ticket is valid on the 8.35, for the very simple reason that you can’t buy one.
If you plug that route into National Rail Enquiries, then at the time of writing it will show you a Super Off-Peak Single for the 12.35 and later services; all the earlier ones are Anytime Singles. So this strongly suggests your ticket will not be valid on an earlier train.
Presumably the earlier trains will not be in the "super off peak" band for at least one of the companies involved, but it can be little tricky to figure out exactly how those are defined . For South-Western, it seems to be "after twelve if not going to London"; Great Western and Southern just seem to say "check National Rail".
I have never tried changing a ticket type like this and I don’t know if you will be able to just pay the difference on the day – I worry maybe not and they’ll just say "you need a new ticket, pay up".
Credit:stackoverflow.com‘