They're not jettisonable or replaceable. What you're seeing is re-use of stock footage of the 2nd pilot version of the ship from WNMHGB to save money in post production f/x.
Well, yeah. I was assuming we knew that.
That's always my answer: IT WAS A TV SHOW.
I was trying to be fanboyish, I guess, and see if people had any mental gymnastics to make sense of what we were seeing.
And they do, apparently: retractable!
The spikes sure seem to be darkened over in TOS footage, don't they? I give the production a mulligan on them, since they seem to have TRIED to make the old footage consistent with the new.