It’s a pity you didn’t act sooner, as you could have received an additional device in the new Blynk IoT, or a subscription credit if you’d chosen a paid IoT account.
I’d recommend migrating sooner rather than later, as there may be some technical issues to overcome - depending on how complex your existing project is. There are also limits on the number of widgets per template in the various subscription types.
Blynk have said that they’ll aim to give at least 6 months notice before they turn off the legacy servers, but as that announcement will probably be made here then it depends on how frequently you visit this forum. You might also get an email, but they can easily go into your spam folder as they are mailing list emails.
Also, it’s worth bearing in mind that is something unforeseen happens to the legacy servers then Blynk will no doubt make all reasonable efforts to get them back up and running, but their priorities will always be then IoT servers and their paying customers - all of whom are on the new IoT servers, so there could come a point where an early abandonment of the legacy system is the only viable option.
Better to protect yourself by migrating to the IoT system as soon as practical, especially if Blynk is used for something that you consider to be mission critical.