score:2
Yes, you can renew early, and have the validity of the new railcard extended to cover the existing time on the old one.
I'm not sure how much in advance you can do it, but there were no problems when I asked to renew mine a few weeks in advance. They looked at the date on the old railcard (not easy - it had faded quite a bit), filled in most of the renewal form for me, attached the old railcard to the part of the form they keep, and then issued me with a new railcard with an expiry date in 12.5 months time.
Upvote:3
When you apply for renewal of a railcard, the form has a specific section which asks for details of previous railcard, including date of expiry.
What is then done is that the new railcard you receive is postdated to the date of expiry of your existing card.
I've done this with my 16-25 railcard in the past, so I know!
Just FYI: In most cases of identities with validity that can be renewed, a similar system of postdating is adopted, as opposed to varying the validity date. This is to ensure that very few errors and frauds occur in the system. In some cases it is also possible for you to directly request for a postdated validity period for various obvious reasons.