Closing out a plan

A small employer with a few employees has a 403b.  Only one person was enrolled in the 403b.  Now they want to have 3 people enrolled with the employer contributing to the plan (not a match, a straight contribution).  I believe a SIMPLE IRA would be great for them.  No ERISA.

That said, they still have the 403b.  What do they have to do to wind down the 403b?  Do they need to close the 403b before establishing the SIMPLE?  What does "closing it" mean?  And at what time could the SIMPLE be established... concurrently with the 403b?  When all the plan assets from the 403b are withdrawn?  When the 403b no longer accepts new money?

Thank you in advance.