1 Jan 2011
This isn't a solution to your problem, but there are a few things you could try.
PAF was based on an early version of Ancestral Quest, and as you've noted, the Mac version was discontinued long ago, and the Windows version is no longer being updated. There are few commercial genealogy programs being made for the Mac, with Reunion appearing to be the most popular one. I use the Windows version of PAF, so I don't really know the Mac world.
As far as updating the gedcom to the 5.5 standard, one thing that you could try is to take the gedcom that you have to a Family History Center and see if they have the older Windows versions of PAF, then use them to import and export the data to update the gedcom. The FHC where I volunteer has all the older Windows versions of PAF. Try calling your local FHC and ask about what's available.
Again, not knowing that much about the Mac world, I don't know whether your version could run one of the virtualization programs such as Parallels which would allow you to use a newer version of Windows PAF. I realize that this sort of solution will not appeal to a lot of people, since it would require getting a copy of Windows to install in a virtual machine, and there are issues with Windows PAF running under Windows 7. But perhaps you have on older version of Windows available that you could use. The Windows version of PAF works fine even on Windows 95.
Getting back to Reunion, you might want to look into whether is will preserve your ID numbers, making it a viable alternative.
This page from Familysearch gives the programs that are currently certified to work with the new online Familysearch programs for doing Temple Ready work, etc. and may give you some ideas as to other programs to investigate.https://devnet.familysearch.org/certification/affiliate-prog...
Good luck, and if you come up with a solution, be sure to post it back here so that others can see it.