Sounds like you pitch them your level 4 emergency support plan....
Service Updates for v4.3 went all the way up to 23. You could try copying the existing MAS over to 2012, then install the SU's, then reinstall all the clients. That might work and buy you time to do the testing. If that doesn't work, then your level 5 emergency support plan pricing kicks in.
Virtualizing would also be an option. I did that when I moved from 2003 to 2008 and I didn't want to reinstall a bunch of stuff.