My experience is not quite what you are looking for but at least it is a related data point for you to consider. I was able to PC my Arrival+ to their Rewards card last summer (before their CashForward product was offered). It did take an extended call but there were a some good things that came out of it:
1) The first CS rep said that I could only PC to the regular Arrival. I asked to be transferred their retention department
2) I asked/told the retention agent that I want to explicitly PC my Arrival+ to their Rewards card --- She said "yes"
3) In addition, I also asked if I was eligible for a SP CLI and if I could also transfer my APR as well --- She said "yes"
4) Things got even better when she told me that she would also provide a 0% APR till May 2016 AND add 1000 rewards points to my account for not ending the relationship
5) The cherry on top was that Barclay also gave me a pro-rated refund of my annual fee.
Hopefully, your PC experience goes just as well 