Then the company which did the hardware installation installed 7.5.2 over 7.5.5. I then re-installed 7.5.5. Amazingly, the system worked fine. The computer is now two to three times faster on nearly all applications. The one exception being Word 6.
Word is about the same speed as before the upgrade.
I have not had any software compatibility problems. I highly recommend the upgrade. Those PowerBook 500-series are great little systems and have some life left. I do suggest a clean system install following the upgrade.