Though this was set up over 15 years ago in a form now ‘write protected,’ how can I make changes to that document without re-creating the document?
Your tag says Writer, please remove Writer and retag to Calc.
Does Tools > Forms > Design Mode not allow you select the controls? If not, can you if you first unprotect sheet by clicking Tools > Protect sheet...? Click the same a second time and you will get a dialog box, you could also try unticking box at top that says Protect this sheet… If it asks you for a password then I hope you have a good memory.