When has a retcon been good, and when really bad?

[West Bromwich Holme]

In your opinion, when has a retcon been good during a franchise, and equally, when has it been really bad?

For me, it's difficult to say, since I've not been aware that much of retcons, outside maybe of Crisis on Infinite Earths, which I've only seen on TV, not read.

Retcons will always be a controversial topic within any fandom as they're tricky to get right, very YMMV, but I felt it's worth discussing here.

[Stellar Colonies]

In DS9, they shouldn't have inserted the real world Klingon redesign into canon. A short joke involving putting Dorn in the TOS makeup and nobody remarking on it would've worked well in the spirit of the episode, and probably ward off some of the inevitable fanwars over makeup changes.

I disliked the exterior redesign of the Enterprise in DIS. All they really needed to do was touch up the design slightly and cast it in modern lighting to have it look fine, and the changes made in the show were somewhat over the top. The interior redesign (aside from the Willy Wonka turbolifts) wasn't too bad.

This post may be an example of doublethink.

[The free romanians]

A good one would be making fallout:brotherhood of steel non canon
Fallout series

[Cannot think of a name]

Battlestar Galactica reboot was far superior to the cheesy 70s original even if I like the cheesy 70s original, Mormon symbolism and all.

[Tillania]

Highlander II - from mystical forces to stupid aliens. I hate it.

[Myrensis]

Really bad example: Cerberus from the Mass Effect games.

In the first game they were a rogue group of essentially Nazi mad scientist assholes who Shepard wiped out.

In the second game they were retconned to being well-intentioned extremists and the galactic Illuminati with the resources and manpower to match the Citadel Council aka the military and economic alliance of most of the major powers of the galaxy.

And the spiral continues in the 3rd game where they surprise! (but not really) turn out to have still been evil all along, and you end up spending most of the game fighting them instead of the Reapers who are busily genociding the galaxy.