Eh hem!
O.K. This is from memory, but I'm pretty sure it is really close.
No person may describe in any required maintenance entry a unit to be
overhauled unless:
Using methods and techniques acceptable to the Administrator, it has been disassembled, cleaned, inspected, repaired as necessary, reassembled and tested in accordance with standards acceptable to the Administrator, which have been developed by the holder of the type certificate ect.
No person may describe in any required maintenace entry a unit as being
rebuilt unless it has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested to the same tolerances and limits as a new item, using either new parts or used parts that either conform to new part tolerances and limits or to approved oversized or undersized dimensions. This can only be done b the Manufacturer.
So I would classify this closer to a rebuild, but I wouldn't use either! I would use the term 'Major Alteration' and this would require a form 337 and a field approval, unless somebody has an STC......

I are A&P mechanic.