Erin Moran declined to appear on the special for personal reasons.
Only four of all the clips shown (minus mass gatherings such as the series finale) featured characters from the show's post-Ron Howard/Donny Most seasons--three with Roger and one with Ashley. No one from that period was talked about or even referred to.
Donny Most is credited as "Don Most."
Henry Winkler: Good night everybody, and thanks again for being a part of our family.
Ron Howard: Here's to happy days.
Tom Bosley: ...Chuck? Is that you, Chuck?!