Firstly, it appears that the migration to Universal Credit will only be complete for everyone by the end of the 2024/25 tax year, and different people are being told to migrate at different times (see e.g. 1, 2).
So it's possible that someone would still need to renew one more time, although from the wording of your question maybe your own migration is already happening.
Secondly, there is some kind of reconciliation process called "finalisation". It appears to be because the credits are based on estimated income and they need to check you got the right amount from your actual income - see e.g. this description of provisional versus finalised awards.
From what I can tell, some people, but not necessarily everyone, have to make a final application to make sure their awards get finalised properly.