As far as Technical issues you'll want to pay attention to what your primary key is for those. If you're using Serial number or MAC address you may end up with duplicate entries after you've run your Physical to Virtual conversion if you're using an automatic P to V process.
Licensing might temporarily show you as being over at till you handle the differences, but it shouldn't be a big issue for the short term.
Everything else works just fine near as I can tell in our own environment.