You can only donate Rumbler Units you have on top of your last upgrade level. Donating your last Rumbler Unit will not affect the Rumbler it is bound to, as Rumblers will stay in your lineup and on the upgrade level forever once unlocked/upgraded.

Remember, however, that the Rumbler Units are needed to upgrade your Rumblers. If you donate all your Units, you will need to collect them again before you can upgrade yourself.