I'm certainly not the last word on this, but I think you are going at this wrong, or at least with the wrong tool for the job. Beat Detective is great for taking a loose performance and making it tight(like to a click track), but its not something I would use to do a tempo change. Next; do you really want/need to change tempo, or is what you should do is create a "tempo map" of what you have so that you can get a click track that rolls WITH the ebb and flow of what you have? If this sounds more like the right approach, search youtube for videos on how to "create a tempo map in pro tools"(without the quotes). It uses Identify Beat and (when done correctly) allows you to have a click track follow the audio you have(instead of trying to change the audio to match a click).
Another possible solution is to try using elastic audio and slip transients to line up with the grid(assuming you can find a tempo that is close, and the "live" tempo doesn't stray over a wide range). In a worst case, you might be best off leaving the original timing as is and play to it
Last 2 cents, it MIGHT be possible to play new parts, use Beat Detective to extract the groove from the original recording and make your new tracks follow that groove, but its not something a beginner is likely to be successful at(I've been trying to get a handle on Beat Detective for years and still don't feel confident on it at all)
