I use SH3 Commander's 'Realistic Career Length' option, so a career like that isn't possible for me. On the other hand, I'm so busy testing stuff that I haven't had a real career ever in any version of GWX. I just look at it a lot.
You might try saving the career to a separate folder and reinstalling it after the change, but I wouldn't count on it working. The above post is correct - you need to do a full reinstall and that changes everything. Maybe just wait until your career is over to do the upgrade.