Any of the three will work--but I'll just note that the 6.5 Creedmoor does NOT just have an advantage beyond 600 yards. High-BC 140-147's in the Creedmoor started at 2750 catch up to typical 140 softpoints started at 3000 in the .270 at 300 yards--and drift less in the wind at any range.
I've hunted a lot with various 6.5's including the Creedmoor, .260, 6.5x55 and 6.5-06, along with the 7mm-08 and 7x57 (which is the same thing as the 7-08 in a different-shape case) and the .270. Have yet to see any difference in how they kill big game, but do know the Creedmoor offers lighter recoil and less wind-drift.
That said, why change horses now? As prairie goat suggested, the present 7mm-08 in an adult stock would work very well.
