Cumulus may well have an eye on the next book to see if the growth of the WRXP format is to continue, all-be-it to see where it finishes. Knowing that it has an expiry date, will listeners still be attracted to it.
From recent history, it seems listening either stays the same or increases after expiration is announced. Some recent examples (and all of these had pre-announced expirations):
In WRXP's last book (2011), listening was about the same as in previous books.
For WKRK/Cleveland, the ratings for it's last book were down. Mind you, the station had been declining in the books for about 6 months before the flip occurred, so this down book seemed to be more of a regular fluctuation.
WZGC/Atlanta is expected to flip to Sports in about a week and since the flip was announced in July, it's 6+ numbers have nearly doubled.
Finally, WKQX/Chicago also recorded a small bump in 6+ for it's final book (2.1, higher than the 1.8-1.9 where it had been meandering for months beforehand), but IIRC, it's 18-34 numbers jumped over 30%.
So, to answer your question, I think it will either see a significant gain in ratings due to wanting to hear it's last moments or listening will be "business as usual". I don't expect to see a listeners exodus until after the flip.