Answer: Yes and No
You can use Ti backup, then format sys&data, install CM, then Ti restore.
I've done this before when going from ClearRom to Omni.
HOWEVER, this is still quite annoying since some settings and data may likely not be properly backup/restored.
How can we install CM11 over on top of Omni, without the extra (and error-prone) steps of backup and restore data?
I was just about to ask the same question, then xda found your post for me.
I have a Note 2 ATT, and recent updates of the Omni became power hungry and eats up battery faster than ever before, plus, each time the Omni updates, I lose Gapp and SU. Often it gives me a hard time to get SU back.
So I want to switch to the newly published CM11.
I've spent the past couple hours doing this.
First I tried just format 'system' then install CM11, it boots, but gives me infinite loop on "cyongen account has stopped".
Then I formatted 'system' and 'data', CM11 boots without error. I restored 'data' from recovery, the error loop is back on.
(devik and cache have been deleted each time)
So... is there any way of doing this w/o needing Ti backup/restore?