I know HD was an ABC show for it's entire run so it's confusing to me as to why CBS is now in charge of all the DVD releases etc.
How did this happen?

Because Happy Days was originally from Miller-Milkis/Henderson/Paramount, and Paramount was much later purchased by Viacom/CBS. This is why, outside of the first release, CBS has done the other releases.

I was going to say voodoo dolls.

Pretty much.

Doesn't matter what network a show airs on, it's the production company that counts.

Star Trek aired on NBC (and later in Syndication), but CBS now owns Star Trek for TV.

Buffy The Vampire Slayer aired on WB and UPN, but Fox owns the show.

House aired on Fox, but is owned by NBCUniversal.

And so on, and so on.

Pretty much.

Doesn't matter what network a show airs on, it's the production company that counts.

Star Trek aired on NBC (and later in Syndication), but CBS now owns Star Trek for TV.

Buffy The Vampire Slayer aired on WB and UPN, but Fox owns the show.

House aired on Fox, but is owned by NBCUniversal.

And so on, and so on.

By the same token, Hart to Hart was from Rona II IAW S-G (1979-82), then from 1982-84, Rona II/S-G/CPT. The CPT part is why Sony owns that show now, because Columbia is Sony-owned, both for now and for past productions of all stripes, IINM.

Doesn't matter what network a show airs on, it's the production company that counts.

Except that it's actually the TV distribution subsidiary of a network, especially a network that didn't even originally air the show. It's not about DVD set releases alone, it's about the TV distribution rights of the series itself. What, is CBS supposed to make us deny or forget that Happy Days originally aired on the ABC Network just because they (CBS) currently own the series?

Besides, Happy Days and its spinoffs just wouldn't have made a good fit for the CBS Network during its original broadcast, anyway, IMO.

Because Happy Days was originally from Miller-Milkis/Henderson/Paramount, and Paramount was much later purchased by Viacom/CBS. This is why, outside of the first release, CBS has done the other releases.
same reason why STAR TREK, TAXI and other Paramount shows are on the CBS website.

Except that it's actually the TV distribution subsidiary of a network, especially a network that didn't even originally air the show. It's not about DVD set releases alone, it's about the TV distribution rights of the series itself. What, is CBS supposed to make us deny or forget that Happy Days originally aired on the ABC Network just because they (CBS) currently own the series?

??? It's about who owns the show. Whoever owns the show gets to decide (i.e. hire) who will distribute it. CBS is part of a big conglomerate that now has many branches, including a distribution arm. So they don't have to hire out a distributor; that function can be done in-house. Anyway, a brief history:

CBS created Viacom way back in 1952 as a syndicator/distributor of productions owned by CBS. In 1971, Viacom was spun off into a separate independent company, and under solid management it became a major powerhouse in its own right. So powerful, that in the early '90s, Viacom turned around and bought CBS, its former parent. Viacom also bought Paramount, in 1994, which owned Happy Days. I'm not clear on whether Miller-Milkis ever wholly owned Happy Days, or just produced it on behalf of Paramount. But at some point, Paramount owned Happy Days, perhaps from the very beginning. So when Viacom purchased Paramount in 1994, it therefore became the owner of Happy Days.

Around 2005, Viacom, as owner of both Paramount and CBS, created CBS Television Distribution, which I think still exists and manages distribution of all the television properties under Viacom ownership, which means all those former Paramount productions.

Incidentally, Paramount itself had a long history of mergers and acquisitions. It was originally formed as Paramount Pictures back in 1912 and became one of the major motion picture producers. In the mid '60s, Paramount Pictures was bought by the Gulf & Western conglomerate which, in 1968 also bought Desilu Productions and Desilu Studios. Desilu had been formed in 1951 by Lucille Ball and Desi Arnaz and grew rapidly into the biggest producer of television shows: In the late 1950s, Desi Arnaz purchased the old RKO Studios, which enabled the further growth of the company. So when Gulf & Western purchased Desilu in 1968, they were therefore acquiring the former RKO Studios (now called Desilu Studios), and all those shows created by Desilu. And with this purchase, Gulf & Western changed the name from Desilu to Paramount Television (as it already owned Paramount Pictures). So all those shows produced by Desilu were now owned by Gulf & Western.

Over time, Gulf & Western divested many of its properties (which were very diverse and included sugar plantations), and ultimately its major properties were Paramount Pictures and Paramount Television. So Gulf & Western decided to ditch its name and renamed itself Paramount Communications in 1989. And five years later, Paramount Communications was bought by Viacom.

And there's also Filmways, a company formed in 1952 that later created hits like The Beverly Hillbillies and its spinoffs. Filmways was purchased by Orion in 1982, and Orion was subsequently purchased by Viacom in 1986.

So that's how this diverse range of shows that aired on different networks over time ended up under the CBS Distribution umbrella -- from I Love Lucy, The Untouchables, The Lucy Show, Mannix and Mission: Imposssible (Desilu productions that aired on CBS), to Star Trek (a Desilu production that aired on NBC) to The Beverly Hillbillies, Petticoat Junction, and Green Acres (Filmways productions that aired on CBS) to The Addams Family (a Filmways production that aired on ABC), to all the later Paramount Television productions like Happy Days and Laverne & Shirley, which aired on ABC.

There have been numerous mergers and acquisitions over the years, so it's much more than just the Filmways and Desilu/Paramount shows that fall under the CBS Distribution umbrella.

http://www.cbstvd.com/shows_main.aspx