10-23-2009, 01:11 PM
I don't believe there are any problems in well over 90% (maybe as high as 95-98%) of cases in just doing an upgrade. There will be the rare case, as in mine. After the upgrade I booted to a kernel panic and couldn't get into SL due to a piece of 3rd party software I had.
I would recommend using SuperDuper! to create a full bootable backup onto an external drive - the choices for this are almost endless nowadays - but I personally prefer WD drives. I also use an external quad case from OWC instead of an external kit from WD - it is more expensive and not necessary for most people.
This is what I had prior to my upgrade. Upon realizing I couldn't boot into SL, I simply rebooted the machine into my external SuperDuper! Leopard backup, opened up the error logs on the internal drive, found and removed the errant application causing the kernel panic and then booted up into SL, no problem.
This will also allow you time to make sure all the apps you require are fully functional in SL. If you find there is something not compatible with SL that you must have, simply boot back to the SD! backup and now do a reverse backup from the external to the internal. In less than an hour you're back to running Leopard again.
You now have the time to track that app from it's developer to see when they'll have it upgraded to work in SL for another try.
One thing to note, after creating your bootable backup, make sure you boot to it and verify that all is ok and it's functioning prior to doing the upgrade on your internal drive.