"[A] plan with a grace period must eliminate it in order to adopt the carryover.... On the one hand, the grace period allows more than $500 to be carried over, but only for a limited time. On the other hand, the carryover is capped at $500 but can be used up over an entire year.... [A] plan with a grace period that wants to change to the carryover for 2013 account balances will have to act by the end of this year -- and explain the change to participants who had been expecting the grace period at the end of 2013. Action in 2014 seems more likely as employers ponder their choices."