I think you have a couple of choices.
If your PPL is still valid then you can carry on using the original 61.75 airman's certificate.
If it isn't, you will have to get a new 61.75 private issued on the basis of your CPL. To get CPL privileges, you will have to do exams and a checkride. This does mean getting the verification of your CPL sent to the FAA from the CAA. TSA clearance won't be required unless you want to do training (BFRs and hirer checkouts are OK without TSA).
There's nothing I'm aware of to stop you maintaining both PPL and CPL. I have a lifetime UK PPL and a JAA CPL.