Welcome to the forum. Short answer: Yes, they can be opened from within Finale.
Please download the 27 demo and check it for yourself. The Aria playback engine and Garritan Instruments for Finale will not be included in the demo but otherwise, the app is fully functional during the demo period.
Mac or Windows and what OS version?
How your files will display and what you need to do will depend on many factors. Please post any issues in this thread and resist the temptation to create new ones for every issue if you have any.
When in doubt, move 2007 to a different directory for awhile
A software is not limited to its folder but includes DLL's, register entries, ... : if the new version modifies these the old one will not work anymore, it should be reinstalled but I don't have the installation pack anymore.
Your Win machine needs to be operating 64 bit to use Finale 27.
"The Aria playback engine and Garritan Instruments for Finale will not be included in the demo" What is it ? "The demo doesn't include the Garritan sounds and Aria player"
Yes, I understood, I don't know what are Garritan nor Aria.
The 2007 version is good enough for what I'm doing with it, the only reason I want to upgrade is that it's 16 years old; I just want to know if I shouldn't rewrite my hundred scores.
Can I send one to somebody who is using version 27 to try ?
It is not a big glaring screen so make sure you read all the questions Finale asks when you are installing the demo or even the full version. I can say to your concern about the registry files, no version of Finale I have installed has overwritten any of them. The older version always worked as they always have. And I have updated every time there has been a new version come out.
I just tried a 2004 file and 27.3 opened if just fine. I am in the process of committing all my music to hard copy so I have been opening some very old Finale files. One problem is if you don't have the font installed on your computer that you used in an older version it may not turn out well.
Following your messages I installed version 27: the 2 coexist and work on my PC. Finale 27 opens old 2007 .MUS files and renames them to .MUSX without touching the old file. So I'll be able to try version 27 and decide if I buy it or not.