Plan is on Corbel prototype document. Plan had no age/service requirement for eligibility, but is amended to add age 21/1 year of service. There was no special provision in the adoption agreement to grandfather employees.

If a participant is under 21, does the amendment remove the employees eligibility until age 21 or is his initial entry preserved?

If participant was part-time never working 1000 hours, does the amendment remove eligibility for the employee?

Eligibility is not a protected benefit, and therefore a plan's change in eligibility standards could cause a current participant no longer to be an active particpant. That being said, pre-amendment eligibility can be grandfathered by the plan document. I believe the default in the Corbel IDP is grandfathering, but the prototypes do not automatically grandfather (unless elected in the AA).

So, if your examples, unless you elect otherwise in the AA, someone not age 21 will no longer participate until age 21 (and the entry date is reached), and someone who never completes 1,000 h/s in a plan year will no longer participate.