Distinguish reversible and irreversible modifications
Before modifying vintage gear, ask what would actually be required to return it to its previous state.
A plug-in replacement or a part using existing mounting points may be genuinely reversible. Routing, drilling, cutting, refinishing, circuit-board changes or cabinet alterations can leave permanent evidence.
Reversibility must also be practical: if restoration requires unobtainable parts or invasive work, the modification is less reversible than it first appears.
Always document the condition before the work, the intervention itself and any removed parts.