I have several computers sharing a kmsync file, which is hosted on my 2010 Mac Pro. I have just upgraded that computer to Keyboard Maestro v9. The other computers are running v8.24. Can v8.24 and v9 share a kmsync file? If not, will there be any problems when I boot those other computers? I plan to upgrade them, but they are running v8.24 now so they will hit the kmsync file at least briefly before I get them upgraded.
When you boot the KM8 Macs, KM will auto-detect that the sync file is for a different version, and offer you a choice to upgrade or disable syncing.
I've gone through this several times, and it has always worked perfectly.
Just remember, your macros will get out of sync from that point on, and eventually if you get them all back to the same version, one of them will overwrite the others. And you wont know which one. So basically any change you make to your macros while the versions of Keyboard Maestro are out of sync may be lost later.
If you are not going to update all copies of Keyboard Maestro at the same time, before making any changes to your macros, you should turn off syncing, recognise the state you are in, decide on a specific Mac to be definitive, and then eventually turn on syncing again.