That is dangerous advice. They may fit on the same rims, but they aren't at all equivalent tires. The 295 has about 700 lbs more carrying capacity per tire than the 275, which means you could have axle overload if you changed from 295 to 275. And the 295 gives 503 revs/mile vs 518 for the 275. That will give you a 2.5% difference in speed and odometer readings. The 295 is a wider tire and requires more dual spacing.
The 275 is a smaller tire in all respects, so don't go swapping your 295s for 275s without calculating all the effects of the differences. Michelin sells the 295/80 size as well as the 275/80, so clearly they know about the difference.