From leopard to tiger, and then back...

I recently installed leopard on my macbook, and it has worked fine. Yesterday i tried to install some software that would have worked on tiger. I was wondering if it is possible to go from leopard to tiger, and back. I also want to know if the program would work.

what is the program?

Its Diablo 2, and the only reason i thought if i downloaded it in tiger then reinstalled leopard it would work, is because warcraft 3 works and i am assuming i wouldn't be able to install that in leopard.

I would like to know if there would be any problems, and if it is pssible to do it without an external harddrive.