If a 401k plan is amended to add an In-Service distribution provision and the provision is utilized by at least one participant, can the plan later be amended to remove the In-Service distribution provision so that no other participants may receive an in-service distribution?

See Sieve's excellent post here about to what extent this can be done.

It may be a longshot, but ... if the provision was implemented with a fixed temporary timeframe, it may have automatically expired. (Sort of like an early retirement window.) However, such provision should have been non-discriminatory at the time it was created.