Mid-year Safe Harbor Amendment - Compensation

Compensation currently defined as full year compensation.  Plan sponsor wants to amend Safe Harbor plan to use compensation from date of plan entry, effective immediately.  Calendar year plan with semi-annual entry dates. 

Since it is March, the amendment would affect only employees who are not yet eligible for the plan.  I would generally apply the same principal of the IRS approved example of changing entry dates...if it applies to future participants only, it would be permissible.  Do you agree?  Or do you think the fact that the definition of compensation applies to all participants causes us an issue?