voipin, keep in mind that the compression licenses do not go away with SIP trunks, those licenses are for the system (voicemail, eMOH, etc) and any TDM endpoints.
In other words, if you have 2 3300's connected an IP trunk, with and ONS phone on each, you'll need a compression license on each 3300 to enable G729 across that trunk when those phones are talking to each other. Changing that to a SIP trunk doesn't change the compression license requirements at all.