The FMs hit downtown just fine, but there’s bad multipath in an area extending from SODO to the downtown area. HDRadio does a decent job of fixing that and for the talkers on FM turning off the stereo pilot helps a LOT.
As for the AMs I don’t think there’s any pressing need to “replace” them with FMs. Two of the stronger AMs are simulcasting and a third (710) does just fine without a FM simulcast. Other weaker AMs are already being simulcast on HD FM subchannels.