Although I didn't get a reply I removed and installed the new fan last night. I'll give a basic derscription of the job for anyone who might have to do this themselves.
The old fan is fastened to the roof with six screws. It was sealed under and around the base flange with vulkum only.
The replacement fan has eight screw holes, only one of which lined up with an original screw hole. This required drilling new holes and finding a two more screws of roughly the same size.
I put a bead of vulkum on the bottom of the base then screwed it down part way, and then injected another bead under and around the perimeter, some of which squeezed out when all screws were evenly tightened.
Although the original fan didn't have any sealant gooped over the screw heads, possibly becuase they are hidden under the pop-up cap, but I sealed my screw heads just to be sure.
I haven't installed the new interior trim ring yet but I made a temporary power connection and the new fan runs much quieter than the old one. The new fan also has a little push button on/off switch, which I considered removing, but didn't. I'll just leave it "on" at the fan and use the wall switch as with the old fan.