2) You partitioned /tmp on a raid. replace 1 drive at a time with the new partition drive. Once all drives are replace look at man mdadm for resize command.
3) You partitioned /tmp on a /dev/h(s)dx. Replace the drive and repartition.
4) You did not partition /tmp on a seperate /dev meaning that /tmp is still part of /. Backup everything and start over.