Am I correct that you have set this up so that you have just one record for each part?
If so, you'd need to use a script to do the requested update. Either for the one record you just updated or replace field contents can do a batch update. The script would copy the value from one field to the other and also clear any fields that would then need to be cleared.
For inventory management, I suggest maintaining a "log" of each inventory change. That "history" of how, when and why your inventory level changed--including but not limited to adjustments made via "cycle counts" is often very useful to people responsible for managing inventory levels and any needed re-ordering. Such a method creates a new record in a "transactions" table each time an inventory count for a given item changes. Sorting and summary calculations then show the current "on hand" counts--and a script can copy this value back into your inventory table after each update if you need faster performance when accessing these counts.
If you are interested in that approach, take a look at this thread: Managing Inventory using a Transactions Ledger