Plan A is merging into Plan B. Plan A's normal retirement age is the later of age 60 or 5 years of service, while Plan B's NRA is age 65. Plan A's employer contributions are subject to a graded vesting schedule, while all contributions to Plan B are immediately vested.

After the merger, must Plan A's NRA be preserved for Plan A participants? Would it make any difference if the Plan A participants are given full vesting in their pre-merger Plan A contributions?

I am not sure that normal retirement age is defined as a protected benefit. However, the normal retirement age is used for 100% vesting, time of payment requirements , and in db plans, the amount of benefit and funding requirements. To the extent the change of NRA impacts one of these, there is at least an issue to review.

I believe that plan A's NRA must be preserved for the plan A portion of the benefit.
But you have to look to the document to see what happens next. NRA (as defined in the Code) is for the purpose (primarily) of setting 100% vesting, as RTK states. Look to the plan for its definition of NRD.