Anti-cutback rule only protects accrued benefits - not the right to accrue future benefits. So, if you have immediate eligibility, and you amend for the next year to be one year of service, then those folks who participated previously but do NOT have a year of service in any year will not be eligible to participate. You CAN grandfather in existing participants, but aren't required to.
Of course, if they have already satisfied a year of service, then the amendment won't exclude them. You'd have to find a valid exclusion classification to then exclude any such people.
I see ETA replied while I was typing...I type very slowly with all these extra thumbs.