Hotel won't honor last year's promotional price because I've rescheduled my stay

When you booked that room the first time, for the promotional price, you booked that date and THAT was a booking. Fast forward to your modifications being made, you've now made a NEW booking. Technically. The person working the front desk is probably just using the system they have in front of them and doesn't plan on getting in trouble just to make you happy. I recommend scheduling a conversation with the manager of the hotel and discussing it with them. Hotel managers have a lot of control over the price of a room. But they will not budge if you come at them sideways. If they refuse to help ask them for the contact information for their boss. Etc. But just stay polite about it all along the way and you have a pretty good chance of success.

It heavily depends on the exact terms and conditions given to you.

But I find it rather unlikely that you were provided with a "as long as you tell us 30 days in advance you can reshedule to any other date for the same price" clause. More likely you have "30 days notice to cancel or reshedule" clause, which does include the option for the hotel to charge you for any price differences.

Let me give you two examples:

1. You book the hotel for dates in the off-season but reshedule to a date at the peak (where prices are easily 2x to 3x higher).
2. You book the hotel for the high-discount and reshedule your room to someday in the future (as long as do so 30 days in advance)?

Both things would ruin a hotel if they allowed their guest to do that.

Your best option is either to honor the booking as you made it, or ask for a refund.

I feel like I am being taken advantage of;

Did you read the terms and conditions of the promotional booking before you committed to it? Most likely, the hotel is abiding by those terms and conditions, in which case the best way to avoid feeling taken advantage of is to read them when you commit. I know this sounds flippant, but I mean it in earnest, and I am speaking from experience. The negative emotional impact of this situation would have been far smaller if you'd informed yourself from the start.

Of course, it's also possible that the hotel isn't following the terms and conditions. In that case...

do I have any legal grounds here?

The best way to know what grounds you have or do not have is to read the terms and conditions.

There is one other very important point to keep in mind, however. Not everything in the terms and conditions is necessarily legal. Consumer protection law often places restrictions on consumer contracts; any term of the contract that violates one of these restrictions is typically null and void. This varies widely from one jurisdiction to the next, and it can be time consuming to research, but if there is a consumer advocacy organization in the hotel's city you might want to run the story by them to get their take on it.

Furthermore, if the company told you verbally something that was more favorable than what was written in the contract, the more favorable term could actually be a part of your contract, though it will be difficult to prove in court what they said. My approach in these cases is to argue to a customer service manager that the company has a duty to train its employees properly, that it didn't fulfill that duty, and that it should honor the promise even if the written contract says otherwise.

The answer will depend on the specific T&Cs of the promotional rates you booked, but generally speaking, no, you don't.

The hotel gave you a discount for specific dates, but you are not staying on those dates, so the hotel has no obligation to offer any discounts on different dates. If anything, many of these "100% prepayment" deals are completely inflexible, so you're lucky they're allowing you to use the money as credit for a different stay.

For what it's worth, stay dates for promotions are often limited to specific periods, so it's quite possible that you would not have been able to get the discounted rates for your new dates even if you had booked earlier.