http://www.changeofstatus.com/resources/regs.asp
Treas. Reg. 1.125-4, ©(4), example 8.
Your documents need to state how you are going to handle these situations. Basically, terminated and rehired within 30 days has one set of options which are: new election (which is only permitted if the facts and circumstances support it); reinstate the old election (step back into) or require they wait until the next open enrollment (lock out). This is to prevent someone from quiting and returning in less than thirty days specifically to change their 125 elections. The "step back" or "lock out" are recommended as the easiest to administer and to prevent potential problems with the facts and circumstances.
Terminated and rehired in more than 30 days is treated as a new hire, who must meet the regular requirements of the plan, waiting period ect.