Migration from v2.1 to B3 requirement for withdrawals

I am not talking about V3 in itself; I know this was discussed in much detail. I am specifically talking about disabling withdrawls on V2.1 and forcing users to migrate to V3. I’ve just re-read the part of BIP15 on the migration and cannot find anything on that. That action came as a total suprise for me (and I guess many other users).