Backup your current Library (just in case)
TRY
Right click the Library Icon: Switch Create: Use Existing: Point it at your Old Library. (it should show if it was not Read only)
Preferences: Adding Books: (adding process tab):
Tick and Set When using Copy to Library preference
Select just a few books: right click on them: Copy To Library: <Present Library> (the
few is so cleanup is simpler if it heads south)
It will offer to create any missing (or different type) Columns in the destination.
If youse happy, do the rest
Use Quick Switch on the Library Icon to return to the Main Library