I would treat this as a count-transfer worksheet rather than item-by-item edits. First export the Item Library and save that untouched as the rollback copy. Then make a transfer sheet with SKU/variation, current qty at location A, current qty at location B, move qty, new qty A, and new qty B.
If Square Plus still lets you import inventory values, test with one low-risk item first: the New Quantity/location columns replace the count, they do not add/subtract. Once that test behaves, batch the transfer by importing the recalculated counts.
If import is blocked for your plan, the same sheet is still useful as a control list so both location edits stay paired and auditable. I would also ask Square support to confirm whether your current Square Plus plan should expose Transfer Stock/Transfer Orders, because the plan migration language and the visible dashboard controls may not be aligned for your account.
