Probably a dumb question, but anyway. Would it be safe enough to just change my wallet passphrase, then claim the forkcoins, and then change it back?
No. The wallet passphrase is only required if you are needed to unlock a given wallet file.
If the attacker has your private keys... they don't need your passphrase at all. By importing your private keys into other (somewhat) untrusted forkcoin wallets to claim these coins... you are potentially exposing your private keys.
This is why you are advised to move your bitcoins to a completely
new wallet (with a new seed/private keys)... that way, when you import the old seed/private keys, there is no chance your BTC (obviously the most valuable) can be stolen.