Everybody with SW 2003 has gone through it, more than likely. You DON'T want to make the jump DURING a major project.
You DO want to make BACKUP's of ALL your SW part, assembly, drawing files, since the switch WILL make the files no-readable with 2001, in case you need to go back.
We didn't have any major problems, but some folks have. A lot of folks do NOT care one bit for 2003, but I think it just appears to be very bad, as most people that have no problems, are not going to complain about them.
Read the Maintenance Notes on each and every SW 2003 Service Pack. Might help you if you see something that you know will cause you a problem. If it does, then you can go one SP notch lower.
Mr. Pickles