Did you need to reinstall FCS after upgrading from Tiger to Leopard?

You supposedly can do it, but it's not recommended that you do it. There are a lot more problems that can arise if you try to load the new OS in the background. I would wait if at all possible.

Don't even think about it unless

#1 You MUST have a new OS to complete the project (i.e. new plug-in only works in 10.5)

AND
#2 You'll have a lot of extra time on your hands to reinstall everything when FCS doesn't quite work the way you need it to.

Of course you're familiar with the recommendation to clone your system before ANY upgrade.

Please don't do this. Finish your project first.

There are radical differences between 10.4 and 10.5. Unless you do the upgrade properly, with an Erase and Install, your next topic here will be to ask why FCS no longer works after upgrading the operating system.