I don't know anything about the MAC or it's disk format.Maybe you could send a GEDCOM file by e-mail to yourself from the MAC, then receive the letter on the Windows machine and import the GEDCOM file to that system. Import it to the family file there and have it all taken care of.
I wouldn't worry about a paper backup, GEDCOM file on a floppy disk, or any removable media should survive just as well.
There should be no problems with the files when the calander rolls over. The only problem I can forsee is that some NUT could unleashe a virus on us.