What is required to update a document created with Framemaker 5.5 to a FM version that will run on Win8.1 & Win10?

I second the suggestions made by Arnis.

The general guidance is that any newer version of FM can open the binary (.fm) files created by any older version. If the version gap is large, you may get some dialogs. If the old .fm has any corruption, random issues can arise. The biggest problems I've run into with archival .fm's are imports that moved or are lost.

If the old (or an older) version of FM is handy, saving as MIF to .mif in the old version provides cleaner data structures to work with. If the previous authors were attentive stewards, they might have saved MIFs at the time.

Conversely, older FMs cannot open .fm files from newer major FM releases (well, you can try the Heroic Open trick, but set your expectations low). Older FMs can open newer MIFs, but will ignore all tags they don't support, and probably toss dialogs on any import formats they don't support.

FM can also save as binary to one release back. Presumably, newer features are omitted.

There shouldn't much of an issue going to the latest version of FM from 5.5. I would recommend saving as MIF and then converting the MIFs back to FM binaries. Also, don't try to convert the book file. Re-create it in the version of FM you will be using after converting the MIFs.