Union said that last yr 20% of new hires had some form or retraining or extra training. Not a big deal in my opinion some just want a few more hrs of IOE or another sim. They'll get you through if you have a good attitude and are trying.
As far as having previous experience in a 737 before showing up; sure it's a pita to learn the SW way and some of it, ok most of it, has you scratching your head but it's their company and their way. No big deal. Everyone I flew with appreciated my previous experience so it can't hurt