I've seen a few of the old series episodes on H & I channel. Noticed that someone has tracked in some CGI in certain scenes. First noticed it during an outside shot of the Enterprise as it orbited a planet. The planet was most definitely CGI'd (stunning detail of clouds/seas/mountain ranges) and I think the ship is too. The views via the bridge's external cameras and background landscapes, too.
I'm guessing the primitive special effects were deemed to archaic to hold a modern audience, but I think they should have just left it be.
When my children was small, I recall that a Turner owned channel carried the old Tom an Jerry cartoons. It was pretty much undisturbed, but one glaring change I noticed was that they'd changed the housekeeper's voice. Originally, she had a voice similar to Butterfly McQueen. in the washed version she sounded like a white woman from the suburbs of LA. I'm guessing some white guilter working for Turner figured the character's "mammy" voice too offensive. Don't know. Like the old Star Trek episodes, they should just leave the old stuff alone.
It's like someone found George Washington's personal writing desk at auction and decided to refinish it with a fresh coat of stain/urethane to spruce the old scratched thing up and trashed his slave ledger found in the drawer so as to not offend.