4.X Binaries on 5.X or 6.X?

[Don O'Neil]

Are there any reasons why I couldn't run any userland apps that were built
on 4.X (some on 4.6, some one 4.11) on the 5.X or 6.X versions of FreeBSD?

I'd like to take advantage of some of the newer core system changes, like
better SMP, etc... But I'm afraid I'd break everything that I'm running, and
some of it I can't re-build for 5.X or 6.X because I don't have the sources,
and the company I bought it from is no longer in existance.

Thanks!