Welcome to the forum!
Altering a railcard, and then using it to fraudulently obtain discounted travel for a sustained period of time is a very serious matter and you should be prepared for this to end up in court. If you have friends who are doing this get them to stop now. I would buy a 16-25 Railcard at once and use this to obtain discounted travel legitimately.
The good news for you is that you are dealing with Govia Thameslink Railway who are the most pragmatic train company when it comes to dealing with this sort of thing. What happens next is they will write to you, saying that you have received a report and are considering prosecuting but also asking for your version of events before deciding how to proceed. You must reply to this letter and I suggest you include the following points in your reply:
- That you are sorry for what has happened
- What you have learned from the incident
- That you are keen to settle the matter without the need for court action
- Offer to pay the outstanding fare and the train company's administrative costs in dealing with the matter
GTR will normally offer an out of court settlement to people who co-operate with their investigation and who haven't come to their attention before. The problem is you have come to their attention before, and this is going to make it much, much harder to keep the matter out of court.
If they offer you a settlement then expect to have to pay the full Adult Anytime fare for all of the journeys made with the fraudulent railcard (i.e. since you turned 18) with no credit given for the invalid tickets you did purchase. In addition, they will want you to pay an admin fee, typically £150. You will need to be in a position to pay the settlement in full within a few days of it being offered, payment in installments isn't normally allowed.
When it arrives post a copy of GTR's letter in this thread along with your draft reply and we will proof read it for you. DO redact any personal details before uploading.
Finally, if you haven't already done so you should tell your parents about this. Given the potential consequences of an altered railcard, and the fact that you have come to GTR's attention before, you might want to consider using a solicitor to assist in negotiating a settlement, either now or if your reply to GTR doesn't result in a favourable outcome.