The forks are only changes in the rules, the keys and the addresses are the same. So if you've got money that hasn't moved since the fork, then any transaction

The forks are only changes in the rules, the keys and the addresses are the same. So if you've got money that hasn't moved since the fork, then any transaction to that money is valid in all forks. Only if different forks accept different transactions to the same account does this stop working. I haven't looked into how people avoid this, I suppose you could bully people by listening on the fastest chain for transactions and then rebroadcasting those to slower chains, in theory you have some chance of messing up their shit.