I upgraded from a S6 to a S9 and I intended to do the same but the battery charges so slow with the USB-A cable that I ended up using the USB-C, probably because of the bigger battery. I would assume that it would be even worse with the AWU and it's huge battery.
Being completely honest: just buy another power brick. Fast charging is something you think don't need until you do.
I upgraded from a S6 to a S9 and I intended to do the same but the battery charges so slow with the USB-A cable that I ended up using the USB-C, probably because of the bigger battery. I would assume that it would be even worse with the AWU and it's huge battery.
Being completely honest: just buy another power brick. Fast charging is something you think don't need until you do.