Well, I just deleted the .DPROJ and the rest of the binaries (DCUs, RSM) and loaded the .DPR. That makes Delphi to rebuild the .DPROJ file with the bare minimum contents.
Once loaded, I added the Win64 platform, set the Release mode and the project compiles right away, no errors.
I upgraded to Delphi Rio (10.3) and installed the corresponding DOA release, but when I try to recompile a project (Win64/Release) it fails:
[dcc64 Fatal Error] DM_DOA.pas(6): Bad unit format: 'C:\Program Files (x86)\Embarcadero\Studio\20.0\DOA\OracleData.dcu' - Expected version: 33.0, Windows...