I am in the process right now, so this is what I have done so far.
I was offered my 18 year contract, I didn't want it so I signed no, etc That started up some other paperwork and I said I would like to transfer into the P Res after my release. I selected a Reserve unit I wanted to join, contacted them and after the RSM says yes I will get a position number which gets me a spot in the unit upon my release. I am still waiting out for the RSM approval, so I am not 100% sure on what happens next after getting a position number.
Since you wish(I assume that is what you want to do?) to VR from the Regular Force and join a Reserve unit it may be a bit different. My friend who did it wrote up his VR memo and said he would like to join a Reserve unit and at some point in the release process the unit said they would take him.
Best course of action is to contact the Reserve unit you wish to join and start speaking with them. They will be able to give you more info, as well speak with your clerk as they will be able to get more info as to what you need to do now.
Also, the BPSO has nothing to do with this... I know because I called their office and the person(wasn't the BPSO, perhaps it was a clerk?) on the phone pretty much made me feel like an idiot for calling them about this as they do not have any part in the process of going Reg-Reserve(even if you are changing trades)