Is a 6.88 mm. cartridge the same as a .270 cal.?

The 6.8 SPC is indeed .277" groove diameter, the same as the 270 Winchester, if that's what you're asking. And it has been used for deer. That isn't to say it's a good idea, though. The 6.8 SPC can't handle bullets of adequate sectional density to make it a good caliber for deer hunting, and the basic problem is simply that the AR 15 reciever is just too small to chamber any good deer cartridge. I've never quite understood why anybody would want to burden himself with the handicap of using an autoloader for deer hunting, but if you must you should look at the AR-10 platform.

To change .270 Winchester to 7mm Remington Magnum, you'd have to change the entire gun. Just buy another gun. But one question- other than just wanting one, why the heck do you need a 7mm Rem Mag? Unless you're hunting elk, it's not really worth it. Your .270 Winchester will take just about anything in North America, other than perhaps moose or brown bear.