I'm curious, did you wind up having to convert your off-air to QAM? I'm still stuck between a rock and a hardplace when combining analog cable with broadcast 8VSB. For my accounts with modest channel lineups, I'm going to move their analog SMATV to vacant UHF channels.
No. Actually I did two different things at two different properties (both are townhome developments).
The systems in my properties are owned by the developer. I wanted to add the digital locals, but knew I wouldn't be able to get it (investing in the equipment) past the developer, which meant it would be on our dime. The lowest channel I had to worry about was 28, so I bought a 522/542 diplex filter and an antenna. Certainly not the preferred method, but I just combine the antenna with the SMATV, and all is doing well. The highest channel on the lineup is 51, so there's plenty of room for expansion. I threw in a modulator at channel 100 for balancing. Fortunately, the vast majority of our transmitters are at the same location, so signal levels are fairly even across the board. The one (and only one) positive note of that method is that they also get many of the LPTV channels. One of the downsides is that it is a big waste of bandwidth. Hopefully, next spring I'll get to go MFH2 in this property.
The property where this gear came from has suspended construction indefinitely. There are only fourteen homes (townhomes) and the developer (owner of the system) couldn't justify expanding their channel lineup beyond the 24 channels they had. The homeowners went with Comcast. Again, I was utilizing a diplex filter and combining the DDC/DUCs with the SMATV.