No it's impossible. Once it's 'in the wild' it's done. In addition... it's purely a small configuration change in the node software. It could be changed to above 83 before Core30, or below 83 after Core30.

If core 'reversed' the default op_return size... what happens to the current Core30 nodes? Do they 'change back?' What if some users don't want to?

Ultimately this isn't about mempool filters or policy really, it's about economic incentives. If users don't like certain transactions on the chain, they should pay more in fees to get their *own* transactions mined instead.