Of course, you should always check with your IBM Rep, but IBM BAW is has subsumed IBM BPM in their offerings. That is you can no longer buy just IBM BPM. Your license for IBM BPM should convert to an IBM BAW license.
This article articulates the various upgrade paths. If you have the option of going to BAW I would advise doing so. From what I remember they announced at Think last year, going from BPM -> BAW is straight forward from a licensing standpoint. However if you are coming from Case Manager it is more complex as I think CM had a per seat or per user pricing model, and BAW follows the PVU licensing model (hardware based approach) which is the same approach followed for IBM BPM licenses.
One important note - as of 8.6 the desktop designer has been depreciated. You will need to plan to convert all of your legacy desktop assets over to Web Process Designer assets as the Eclipsed based designer is being phased out. I think that BAW only supports the Web Based Designer (but please check before assuming I'm correct).