Plan provides that participants who separate from service and are rehired within one year will be given credited service from the termination date upon return for benefit calculation purposes. Plan is amended to eliminate the credited service restoration upon rehire (vesting service still applies). Any anti-cutback issues with amendment? Why or why not? Would the answer change if a lump sum payment was made upon termination before rehire occurred?

There are always potential anti-cutback issues with any plan amendment. The amendment cannot "take away" any benefit, or right to receive it at a particular point in time, that has accrued as of the later of (a) the effective date of the amendment or (B) the date of its adoption.

Most amendments will include some generic provision (or possibly a "preamble") that states the amendment will not reduce any benefit accrued as of the later of (a) or (B). It is possible that your amendment already includes the appropriate language.