I might sound stupid here, but, why is it that they had to dub over "Rock Around The Clock" with "Happy Days" in the dvd versions/reruns of the season 2 episodes, but were able to keep the original for the first season? Now, don't get me wrong, I love Rock Around The Clock, but I have always found this choice quite strange.

I don't know, but it's the same way in the Pluto airings. It's super awkward.

It's likely that it's music clearance rights. They probably didn't get the rights to Rock Around the Clock for Season 2's DVD so that's why they used the familiar theme Happy Days fans know and love.

I remember reading somewhere that they would have had to pay for each individual use of "Rock Around the Clock" on the DVDs. They would have had to pay for 23 uses for the 23 second season episodes.

They ponied up and paid for it and most if not all of the original music for the first season DVD. They seemed to be unwilling to do it for many of the songs on the second season DVD for various reasons.

This is what I wrote in my second season DVD review (https://www.sitcomsonline.com/happydaysseason2dvdreview.html):

A few things need to be said about the music on this set. The first thing you'll probably notice is that Bill Haley & His Comets "Rock Around the Clock" has been replaced on this DVD with the season three theme song. The opening credits remain the same, just the theme has been replaced. It really doesn't have the same 50's feel, but I guess if it had to be replaced this was the best alternative. You can pop in your Season 1 DVD for the "Rock Around the Clock" opening. I guess I'm a bit confused about "Rock Around the Clock" and the various versions that they've used. I was told a few months ago that the show used the original 1954 recording for the first and second episodes, but they started to use a newly recorded version that they had made in the fall of 1973 starting with the third episode. The Season 1 DVD set used the 1973 Paramount version. I was told that Paramount had him re-record it in 1973, so they would own it and wouldn't have to pay royalties to MCA (hence, Universal) for every episode. If this is true and Paramount owns the 1973 version, why didn't they use it on the Season 2 DVD set like they did for Season 1?