Just posting a data point, but I called yesterday to upgrade one of my Gold cards to a 2nd Platinum.

The conversion took about 3 hours. I was able to register for the DIGITAL CREDIT, SAKS, and AIRLINE.

Already put through a charge for AIRLINE AND FLR, so I'll wait to see how long it takes the credits to post. I am hoping we may get additional FHR credits on or after September 18.
I think my anniversary for the GOLD CARD was around October or November, but the rep was very clear that I will be charged a prorated fee of $695.00 around 45 days after the conversion.