3.6 and older in parallel?

Thank you. I did, and it worked (sort of). A few observations here.

Initially the PortableApp (3.6) wouldn't open with old 3.5 also open. So I closed 3.5 and worked only in 3.6. I created a score there that I then wanted to back-port into 3.5. I was able to open 3.5 with 3.6 open. I tried Copy in 3.6/Paste into 3.5. That doesn't work. So I exported 3.6 to MusicXML and opened that in 3.5. The system text came up in the wrong place but otherwise it generally worked, and I was able to copy from that in 3.5 into a new score also in 3.5. Only the notes copied over. The other added markings I had to add to the copy. (I think that's expected behavior.)

Comparing appearances of the two: The Edwin font and general engraving in 3.6 have an overall heavier, denser appearance than the FreeSerif font in 3.5. In the title, the 3.6 default font size is 22 points but the text is longer horizontally than the 24 point text in 3.5. I added a part name in 3.6, also at 22 points. Going through MusicXML into 3.5 it came up looking like the same size but reported at 12 points. I thought that strange. The notes look more crowded in 3.6 than in 3.5 because of the increased heaviness. The overall layout on the page in 3.6 is somewhat better, with an initial indent and better vertical use of the page space. The clef signs in 3.6 look slightly better than in 3.5, though few casual observers would notice. Measure numbers in 3.6 are in italic; in 3.5 they are in roman.

I was able to bring my custom workspace across from 3.5 to 3.6 just by copying the file, and 3.6 recognized it. I found a few new items and a couple of new fonts available in the 3.6 master palette that were not in 3.5.

Overall I'll give 3.6 a thumbs-up, though I'll continue to use 3.5 for some time yet before I attempt converting my considerable library (hundreds of scores) to 3.6. I may defer that effort till 4.x is out.