Going from butt rock laden Rock to a (mostly) Classic Alternative format or straight-up Alternative would be more than a mere "format adjustment."
Edit - I see the Rock 100.5 Facebook and Twitter pages have each been deleted.
Given the recent hoopla around Cumulus bringing back the Q101 brand to WKQX in Chicago, I definitely think an upgrade of the 99X brand to 100.5 is a serious possibility.
Of course, it bears mentioning 99X in its current incarnation earns anemic ratings at 98.9, and during the final five or so years of its run at 99.7 FM, the ratings stunk, too. In much more recent times, Radio 105.7 / Alt 105.7, after earning good ratings its first 12 to 18 months on the air, sputtered afterward.
Thus, I'm not so sure the demand in Atlanta truly exists for an FM alternative station. I'm also not so sure the general public would be as excited to see a bunch of retreads who are now well outside the demo target try to bring the 99X brand to 100.5 FM. A good chunk of the Metro Atlanta population either wasn't born yet or lived elsewhere during 99X's heyday, too.