In most ways, newer versions of Excel can run old version macros. New versions simply have more tools available and perhaps could be rewritten more efficiently if you so chose.

There are a few VBA methods that get removed, like the .FileSearch method no longer supported in xl07+. A macro written for 2000 should work in 2007, but you'd discover it didn't the first time you tried and DEBUGging should take you right to the problem code.