Most stations will be able to increase their HD power to the point that they don't interfere with a neighboring signal. If they do, then they have to reduce the power to the point that the interference goes away. Right now, the only stations that seem all that interested are the non-coms and the NPR affiliates.
Curiously enough, at one point, NPR conducted an in-depth analysis of increasing the power and quickly determined that it wasn't a good idea. As you can imagine, this was a pretty good blow to the I-Biquity folks. It hit them right in their proverbial pocketbooks. However, after much arm twisting (I'm sure) and (dare I say) maybe a little donation to the NPR cause; now, all of the sudden, NPR is on-board with the idea to the point that many of their affiliates are operating, at increased HD power, under experimental authorizations. KUHF has been operating that way for quite awhile, now.
It remains to be seen if the commercial guys are going to spend the bucks to do it. For most, it's going to mean a new transmitter; since there is going to be a trade-off in analog power vs. digital. And the question will always be whether there are enough HD radios (tuned to their station) out there to warrant the expense. As as been the case since the inception of the idea; it's the chicken or the egg paradox. The broadcasters won't do it until enough HD radios are out there. And, the listeners won't buy HD radios until enough radio stations are broadcasting HD formats that they will listen to. Right now, the formats that are on, in Houston, suck. Making more HD 2 and 3 formats that suck isn't going to attract any more listeners; even with a power increase. As Sarah Palin put it, you can put lipstick on a pig; but it's still a pig.