Changing bitcoin core password Let's say I have 5 copies of my .dat file on individual flash drives. I download the blockchain and run a full node and have the core wallet.  I copy my .dat file into bitcoin core. I type in the old password and then change the password to a new password. Will someone who has 1 of my old .dat files and who knows the old password be able to access my bitcoins?
Follow up- So the easiest solution is delete the old. .dat files once I have btc core wallet running with the new password?
Thanks
Will someone who has 1 of my old .dat files and who knows the old password be able to access my bitcoins?

Yes. Including bitcoins added anytime afterwards!
As far as I know, the password is used to encrypt the file contents (or at least the private-keys in it). So old passwords are still valid for decrypting old files that were encrypted before you changed passwords.