Hi Laura,
Too late for you, but a reminder to anyone who has not yet upgraded their OS - whether Lion or a Windows upgrade: never upgrade without a FULL backup. Too many things can go wrong, even though they usually don't.
😉
Also too late for you, but as mentioned in other threads and in all tip web sites and blogs concerning upgrading to Lion: anyone considering upgrading to Lion MUST make sure that all (100%) of critical programs on their system show as "Intel" and not PPC since no PPC programs (such as AC 9) can not run under Lion.
Before you do anything, get an external drive for each Lion machine that has unique / meaningful data on it and let Time Machine make a complete backup. (And/or make a clone of the disk.) I recommend an external drive that has Firewire 800 and is 7200 RPM as the speed over a USB 2 drive is significant. Size at least equal to your internal drive, preferably double.
Since you have AC 9 running on your old computer, open every project and save it as a PLA (archive) in preparation for migration to 10 or higher. Using a PLA vs a PLN will make your life much easier, particularly if there is more than a project or two to migrate.
AC 10 can open all of your 9 files, so now I understand why you were asking the question. Rather than revert your computers to Snow Leopard (somewhat major task and possible loss of data/preferences - and a waste of time/effort) - I would
bite the bullet and upgrade at least one of your licenses to 15 so that you can at least run 10 (which you can download) or higher to open your projects from 9 and continue working on your new/updated machines. Keep your AC 9 application folder around until you've saved the AC 9 and AC 9 SE library folders from it since you'll need them. (This is less important if you have created PLA's for every project.)
As you can see in this Wiki article:
http://www.archicadwiki.com/ArchiCAD%20versions
all of versions 10 through 15 can open files in version 9 format.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB