Last October I went from being a full time employee to doing PRN work for the same company. I did so because they hours were too many for my other activities (I am a coach as well) and it was too hard to get time off (I'm in the medical field and everything depends upon pt caseload and Medicare reimbursement)
When I worked full time I got 2 weeks of paid vacation and a couple of personal/sick days. And I was able to contribute to our company's SIMPLE plan. Since I don't NEED to work, thanks to my engineer husband I was putting all my money into the plan. When I told them I wanted to go PRN I thought I would loose all my benefits. I also know that I was loosing a big chunk of change to the tax man. In lieu of asking for a raise at my review I asked if I could contribute to my SIMPLE plan instead (they are effectively giving me a raise anyways since they are required by law to give a 3% match). I was ecstatic that my boss agreed!!!
Since my job now fluctuations I have elected to put 90% into my account until I reach the $12,000 cap for the year. Take that tax man!!!!
Moral of the story: Don't assume that all rules are set in stone. You never know until you ask!