@mandygeorge is correct, the "new" rule builder for classification sets is not yet built/released... so classifications requiring regex rules must still be done in the legacy system.
Depending on how many rules you have to migrate, When you start the process will have to take effort into account, but there is lots of time so far, again, as mentioned you have until Aug 2026.
That said, I have seen a lot of bugs reported with the classification sets, and encountered some myself.
Your existing classifications should all have been "auto" made into Classifications Sets by Adobe... for better or worse... All my legacy classifications are set to a minimum of 2 suites (QA and Prod), and in some cases 3 (QA, second isolated QA, and Prod), all with a specific naming convention for proper identification of the purpose / dimension.... all of the auto created classifications were created as one per suite, with horrible naming. When I attempt to consolidate them, it failed.
This means I will have to delete everything and re-build.. and since 99.9% of my rules are Regex based (so I will still have to do work in the legacy rule builder), I am not doing this until there the new rule builder is released, because I am concerned that I will have to do all that work again once Classification Sets are fully released. And even when it does, I will test in my QA suite only, and confirm functionality, then start the migration on the least critical rules first, making sure they are doing what they should. Then, and only then, after I have confirmed viability, will I start the migration in earnest.
However, this is a choice I make for myself, and only you know how much work and effort is required to migrate your rules.