Just because the current 99.9 license goes away doesn't open anything up for change, because the underlying allocation doesn't go away and still needs to be protected.
I forgot about the allotment angle since this is not reserved band. I know a WACO-FM move-in has been mentioned over the years multiple times from different folks. Some may have been around this forum, but I remember some discussion offline too. I thought it was basically an open secret possibility from what I was told.
But, I looked into this a bit more this evening when I had some extra time...
On the plus side, there isn't an urbanized area issue and I think this could be done outside of a major filing window.
However, from the ownership cap point of view, Austin is showing a total of 42 stations. I think BIA's numbers might have an error. Regardless, you would need 45 stations to move the market to a five FM cap. It's close, but not quite there.
Next, it looks like a move-in on 99.9 is short-spaced in two ways: EMF's co-channel KLKV in Hunt, TX (C3) and co-channel KTXM Hallettsville (A) as previously discussed.
If you make an assumption that a downgraded WACO-FM would need to be at least a Class C2 from one of the West Austin towers to make it worth the trouble (I don't think iHeart would be interested in a Class A move-in into Austin), then it would be short-spaced to KLKV by approximately 25 miles and about 25 miles from KTXM. There is definitely a reference point north of Austin where it could be fully spaced and a directional antenna could be used to accommodate the spacing issues, but it isn't really clean.
So in summary, if iHeart had some help from their friends at EMF, where KLKV was downgraded or moved to another channel (if possible) and EMF changed something like KLVT from Temple, TX to somewhere in-market like Jarrell and they got one more station to to be added to the market count (like KRXT which is now run out of Taylor) and they were willing to accept a null towards Hallettsville, a move-in does seem theoretically possible, but that is a lot of caveats in addition to giving up the most successful station in the history of the Waco market which has historically billed very well for the market size.
On the plus side, there isn't an urbanized area issue and this could be done outside of a major filing window.