IF a move happens, then it will just be such that all transactions before a certain date/time are stored on the Steem blockchain, and after then will be stored on the Hive blockchain
This will be more complicated than just switching to Hive from fork time. It would require future nodes to run a Steem node up to the Splinterlands transition and then a Hive node. But if you go with Hive from the split, then only a Hive node would be sufficient to construct the Splinterlands ledger.
I think if you're confident enough that Hive will be the primary community chain going forward, you should consider switching. Perhaps shut down the splinterlands gameplay, i.e. maintenance mode on the day of the fork. I'd personally rather have a couple day outage compared to a much more difficult time in the future generating the Splinterlands ledger and determining consensus.