If you watch the newly syndicated versions, they have been digitally enhanced in certain scenes, like the Enterprise orbiting a planet, or certain computer displays.
They] technicians did add to it, and the story lines remain true. Basically, they improved on the ‘60s special effects.
I used to order episodes on DVD from Netflix. My Daughter regularly gave me free subscriptions.
On one disk they had a short interview with the props manager. He said their budget was low and they had to improvise a lot of things.
He said they did have one advantage. No one knew what a Star Ship really looked like. For instance in one scene, they simply stuck plastic cup holders on the wall for some effect.
He also said their favorite source of materials was the Paramount dumpsters.
Basically, they improved on the ‘60s special effects.
I remember how the Trekkies were so mad when CBS had announced they were going to digitally enhance the special effects in TOS. This was shortly after George Lucas had bastardized the original Star Wars trilogy by slathering on a bunch of irrelevent extra CG imagery and changed scenes around (e.g., Grito shooting first).
Everybody was afraid that CBS was going to mess up TOS in a similar way. But it turned out that CBS was careful to make only minimal changes to the original 1960 material, with no changes in dialog or shot composition other than to improve on the limited and sometimes cheesy 60s special effects (and to clean up scratches and defects in the original film negatives).
Once people saw the result they were pretty happy and there were few complaints. The biggest changes were made to the special effects of the space-based episodes, for example The Doomsday Weapon (which I thought was particular well done).