The bolt carrier on the REC 7 is specifically built for the Barrett piston system. The bolt carrier is also matched to the lower. So, it's not quite as straightforward as it first appears to change uppers. It is NOT the same as a DI AR converted to a piston system, the Barrett is designed as a piston rifle.
To use a DI upper, you will have to change the bolt and bolt carrier as the bolt + carrier will not work with DI uppers, and may not work with other piston uppers as the Barrett piston and bolt system is proprietary to Barrett. There is a little cup on the Barrett bolt carrier that matches up with the piston rod. So, a different piston upper would have to have the correct length piston rod AND stroke length to drive the bolt correctly.
I had my REC 7 disassebled earlier today, and I'm not sure you could put a different bolt in it as Barrett has incorporated an anti-tilt system into the bolt carrier and lower. Without a standard AR bolt + carrier to test in the lower to see if the Barrett is compatible with other AR parts, I wouldn't assume interchangeability with other uppers.
Likewise, the Barrett upper needs to have a matching bolt+carrier in the lower in order to work properly. While all of the pin holes in the uppers and lowers are standard and will mate up with other AR's - whether they will work without the Barrett bolt + carrier is the real question.