Hi,
Also If pre-conditions are met,and re-INVITE fails, UEs could use the new SDP. This solution will give someone a way to escape charging. For example, if UEs use audio at the first time, then one of them raise a re-INVITE with a SDP carrying both audio and video,after the preconditions are met, then the other rejects the re-INVITE, but they can communicate with both audio and video after all.
No, if they negotiate video and the preconditions are met, they will be charged for video, regardless of what happens with the re-INVITE.
Cheers, Gonzalo