Absorption panels on ceiling..what to expect?

Knowing the type of speakers,the actual size of the room and where the speakers are in the room in relation to the listening position would be helpful.I would look into ASC Sound Panels.They can be installed on the ceiling with a Velcro tape and are very effective at controlling any reflection from the ceiling. Determining exactly where that first ceiling reflection point is a bit tricky,you might start at the half way point between the listening spot and the speaker.They worked for me in a similar sized ceiling and I was not thrilled with anything heavier hanging overhead.Sound panels are light and very effective.There are a variety of ways to install them as the Velcro setup does tend to be a bit brutal on the ceiling finish.Room setup is a balance of sacrifices made for good sound.

If at primary reflection points on the ceiling, I would expect the soundstage to be more biased towards the other directions than currently due to less reflected sound from above. Panels generally absorb higher frequencies better as well, so I would expect the overall tonal balance to be somewhat less bright as well.

If you took some heavy blankets, or something similar, and tacked them to your ceiling, it may give some indication as to what you can expect. If it turns out to be too much, another option would be to texture your ceiling. But that's only if you need to make a small change.

I once tried tacking some of those soundproofing tiles to the ceiling around the first reflection points and, wouldn't ya know, they screwed up the sound. Kinda like Sonex. Made the sound all phasey like.

I would expect a decrease in sound quality. I would expect the space to be dull and over damped.
What facets of reproduction are you wanting to change or improve?