I've been asked to share wallet credentials to participate in mining. Is that a scam?
Its an obvious attempt of a scam. Don't ever do that. What the scammers will do is as follows:
they will make a copy of your key, then you will lock them out to access your key. It won't help because they would have already made a copy.

Changing your password prevents anyone from using that password to access your wallet and keys. It doesn't prevent anyone who already knows your secrets from just creating another wallet with the same secrets.