I would suggest making a CR'd/SU'd copy of the unit to hang onto just in case. Sometimes you might find you need another unit of that class for a map, or more likely, you'll find yourself needing one for a four star challenge or an event map that limits units based on rarity (there's already one map with a 4* challenge that makes gold+ rarity units cost triple to deploy, and I know there's at least one map from one of those gold rush-like dmm events that put restrictions on units that outright prevented the deployment of higher rarity units.