Do I need to create new service requests for each participant that we are changing over to the TPP pricing?
No, the fastest way is to update your charge bands to the rate and TTP codes assuming it is for everyone (i.e. all participants).
If not then a bit more work is required because you need to use the old charge bands for old customers and new charge band for those who have switched over to TPP. If that is the case then create a new service request and then move the visits to the new service request (which has the new charge band) then remove the old service request since this will no longer be used for that participant.