The antenna changed at Senior Road as part of HD conversion as well. The original antenna was a Dielectric panel. The new one is an ERI.
Several of the chiefs of the Senior Road stations have complained about signal issues with the ERI antenna. It seems to have more nulls than the original antenna.
The problems definitely aren't HD related though. I've personally turned the HD transmitter on and off for one of the Senior Road signals in a problem area checking to see if it impacted performance. Not at all.
I have a different experience. I drive Houston / Dallas a lot. There was a time about 8 years ago when KGLK had HD on, then HD off - it went back and forth a few times. HD off, they make it to Centerville. Any time of the day, any week of the year. HD on, they drop before Huntsville. About a 60 mile difference. Because it happened a few times, I was able to determine how repeatable it is. It is repeatable. I don't have some weird setup - Pioneer aftermarket car radio, 31 in ch whip. That big of a change is dramatic, and will affect building penetration in Houston.
As far as KRBE - I notice the difference between now and before. 130 mile local quality signal vs. 80 or 90 today. No more reception to the South part of Dallas because a local 104.1 came on up there. But KRBE is receivable far South of where that would be an issue. Austin reception like a rim shot? No more, I think there are some local 104.1's out that way now. But that incredible signal from earlier is gone - it used to be a point of pride for KRBE engineers. I bet building penetration was great!
There are some HD holdouts in Dallas, they make deep inroads into the North part of Houston. Spotty reception all over town. Dallas FM's with HD - nothing. It takes a skip event.
Bottom line - multiple stations, multiple scenarios, HD and possibly the antenna switchover you talk about are both the only variable, and the changes are not good! If you don't care about range, you darn sure should worry about building penetration which is going scale right along with range. I am not an HD hater - I listen to the HD-2's because they offer formats not available otherwise. But I am an engineer, and know the scientific method. HD is BAD NEWS for FM coverage. That is ANALOG FM coverage AND building penetration. Since you are an engineer, you know about the gain / bandwidth product. You widen the bandwidth, you lower the gain proportionally. That is the basic flaw of the HD system, it is physics, and no amount of wishful thinking will negate a basic principle of physics. This didn't have to happen. If the designers of the system had just planned to supplant and replace services like SCA, RDS, etc. - the entire HD sidebands could have been placed within the existing FM channel - and these problems would not have serviced. One of the good ideas of HD was to make auxiliary services available on HD-2 and up - so SCA, RDS, and everything else would have been better off taking advantage of HD radio, and the cost of receivers for users of those systems probably less than special receivers they use now. But - no - they just mindlessly assumed there would be no consequences from placing sidebands outside the channel on adjacent frequencies, which opened up Pandora's box of IF jamming, reduced coverage, and drops due to adjacent channels coming in on skip. But - of course - only DX'ers complain about such things, and they can be insulted and marginalized - right?! Nope - consumers have reduced coverage, Huntsville and Beaumont lose Houston stations and are left with pitiful local offerings, HD drops in cars, doesn't penetrate buildings, HD-2 drops on the high side of the FM band when a radio tuned to a station 10.4 to 11 MHz below drives by, HD-2 drops to dead silence which is annoying as it is, and so forth. "Great" system here. BAD engineering - sold with high pressure sales tactics (I've heard about it). Sold to a corrupt and inept FCC which rubber stamped approval without enough testing, rolled out at a time when car radios were being connected to inadequate antennas in the name of aesthetics. What word of mouth there is about the system is bad because its not reliable. All people understand is that digital sounds no different than analog because the highly touted table radios have 4 inch speakers and there is no way you can tell a difference on a speaker that small, the potential of HD-2 unrealized because it takes an antenna to receive HD - most consumers leave the dipole off or it is crumpled on the floor with power cords, the AM loop discarded with the box (the home theater people installation people did that to me) - so much for AM HD, and AM in general. HD TV made the picture better. HD radio doesn't sound different to people accustomed to 16 bit iPod quality audio - a big step down from a good analog signal chain - 96 dB signal to noise as opposed to 120 to 130. So there is no difference to a consumer listening on earbuds or through a 4 inch speaker, making HD radio a solution to a problem that didn't exist - a product consumers don't want. The uninformed consumer doesn't even know they need a new radio. And those that do won't put one in the dash because they lose their integrated GPS, backup camera, etc for a 1 or 2 DIN aftermarket radio in an ugly plastic adapter. I could go on and on - but this whole HD effort was ill conceived, badly researched, badly engineered, ill timed, and pretty much doomed out of the gate. Which sucks for me, because I will lose the HD-2's I enjoy when it dies a protracted death due to consumer and eventually engineering apathy.