If you merge a D/C plan "A" into another D/C plan "B", does plan "A" have to be amended for GUST, or is it sufficient to amend plan "B" the survivor plan? Is there any instance whereby plan "A" would need to be amended for GUST prior to the merger?

Restatement of the merged plan is enough. During the TRA'86 restements, I did a number of merged plans, and got them through without fuss. However, i would make sure that your determermination letter application reveals what you are doing.