The supported answer is No, you cannot change this after it has been saved. The field becomes greyed out. You will either need to Void or delete and recreate.
The unsupported answer is you could change the value in the backend for each of the transaction with this issue. This is unsupported as there could be multiple tables that would need to be changed and could have side effects if something is missed. Just a high chance of additional things occurring.
My suggestion would be to bite the bullet and either void or delete then and recreate them so you know you do not have any issues going forward that would need to be corrected down the road.
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