It is always a good idea to have a safety net when changing OS X versions and/or making software changes.
This is what to do:
You'll need a second hard drive to be a duplicate of your System drive and a copy of Carbon Copy Cloner.
Clone your current System drive to the second drive.
After the clone process is complete, change the startup drive to the second drive and test to make sure it can boot and run the system. The testing need not be extensive.
Once you know the second system drive is good, shut down the computer and remove the second drive.
Reboot and make your changes.
If things go wrong, you can always reverse the clone process - ie boot from the second drive and clone it back to your internal system drive.
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If you want to run FPx, you must install OSX 10.9.
Does FCP7 run under 10.9?
Yes, but not all elements of the whole suite work well. For example, the Compressor 3.5 preference pane does not work, which means you have to set Compressor up using terminal line commands (think Unix based structure) Also, there can be trouble with Soundtrack Pro, Cinema Tools, etc.
I find FCP runs just fine on my installations of 10.9. I've moved to Compressor 4.1 and use Adobe Audition for Audio in 10.9. I do have a machine that runs 10.8 where the whole FCS suite seems to run just fine.
The alternative to doing an over write installation is to keep your first system drive intact and to simply start fresh on a new drive. This works best if you have a machine that holds two internal hard drives. A sub variation on that is to partition your system drive and install 10.9 on the new partition. If you go the partition route, you will want to create the clone as partitioning using Disk Utitliy will erase the drive. Whether you have two drives or a drive with a partition, you are creating a dual boot system - one system stays with 10.6.8 and the other runs 10.9.
Good luck
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