I had this happen to me last year, in Rheims in France, when the HIX left the chain at quite short notice.
I had not been contacted by anyone to let me know about any change, I just happened to notice that the reservation had disappeared from my priority club account and that the hotel also seemed to have disappeared from the booking system!
I contacted the customer service centre who also didn't seem to know anything about it, but after contacting the hotel, discovered that it was no longer an HIX.
They managed to book me into the other property in Rheims, which was a Holiday Inn proper. This reward booking cost more points, but they waived this by crediting me the difference in points.
It was fortunate that I had spotted this when I did, as days afterwards this property showed as full for my chosen dates, no doubt with other guests who had transferred from the HIX!
My advice would be, don't hang around and book something ASAP, Before other people beat you to it!