If the disk is still accessible, first ensure to have a synced array running a sync command:
snapraid sync
Copy the big parity file to the new disk
A good copy command is ddrescue:
ddrescue /OLD_DISK/parity /NEW_DISK/parity /tmp/copy.log
NOTE: This will copy the parity file from the old to new location, and in case of errors, it will continue copying what can be copied.
Having a partial parity file, is still beneficial, as it will still allow to recover data as long it uses the valid part.
snapraid fix -d PARITY_NAME
Configure the new parity location in the configuration file.
Run sync again to make snapraid recognize the new disk.
snapraid sync
NOTE: This command will be almost immediate.