Most 403b plans offer several "vendors" or plan providers for investments. 

Participants in a 403b plan have been able to choose between different insurance companies or mutual fund providers. Under existing rules participants in the 403b tax shelter arrangements could transfer their  accounts to other 403b providers tax free.  This is known as a "90-24" transfer and often made sense to get better returns or lower expense investments.  New 403b regulation will limit these transfers after September 24, 2007.

There has been a lot of discussion about the new 403b regulations and changes to 90-24 transfers.

Several recent articles at 403bwise.com will provide info on 90-24 transfers.