A large part of this discussion hinges on when certain starships were originally built, namely the Enterprise and the Reliant.
Was it ever nailed down in canon when 1701 TOS Enterprise was actually built? Is this reliant upon TAS’ “The Counter-Clock Incident” alone? From it, we can assume the Enterprise was commissioned some time around 2245.
We can assume that, if the United Federation of Planets is roughly 100 years old at the time of TOS, and that the Federation has been deploying starships for roughly the same amount of time, that there were many “missing link” starships in the Federation “star service” during that century that we know little about.
There is no direct canon evidence to firmly establish when certain starship classes originated/ were first constructed. There is indirect evidence that at least some starship classes were kept in service for several consecutive decades, maybe even longer. (Saratoga, Lantree, Constellation-class ships, Excelsior-class ships, etc. The Klingons also seem to keep their cruiser designs in service for generations, if not centuries.) We do not know exactly how long the Constitution-class starships were kept on duty, or their Miranda-class cousins. Was the NCC-1701 Enterprise seen in TOS one of the first Constitution-class vessels, thus establishing the age of the Constitution-class and its design, technology and general philosophy-of-use? It would seem logical, but this depends on numerous other factors being established as canon or de facto canon. (Kirk said “There are only twelve like it in the fleet”, very vaguely suggesting that 12 Constitution-class vessels were in operation, indirectly suggesting that, starting around 2245, the Federation began building 12 Constitution-class starships. But this is implicit, not explicit. Since it seems obvious that the Federation must have had other ships — and ship classes — in service prior to 2245, we can assume there were already a lot more than just 12 starships in the Federation fleet, or even that there are more than 12 heavy cruisers in the Federation.)
For what it’s worth, I favor the notion that a starship, particularly a Federation starship, is not a static thing. Instead, starships seem to be organic-like, evolving frameworks that are expected, from the time they are constructed, to undergo numerous repairs, refits (some minor, some major) over the ship’s useful lifetime which could be at least a few decades.
In TMP, Captain Decker objects to Kirk taking command of the newly “refit” Enterprise, referring to the ship an “almost entirely new” and later making a big deal about senior personnel being “fully rated on this design.” This makes it clear that Enteprise’s just-completed drydock operation was indeed a major refit, resulting in an “almost entirely new” ship. It appears that Enteprise was literally torn apart, her components recycled/rejuvenated/replaced, possibly including segments of the ship’s structure. (The “refit” Enterprise obviously has a significantly different silhouette, even though her basic shape is very similar.)
The nature of starship refits, as with the nature of building and/or refitting entire classes of starships, has never been completely illuminated. That Scotty made mention that the newly refurbished Enterprise has “not even been tested at warp power” after “eighteen months of redesigning and refitting” resulting in a ship with a clearly entirely-new engine room, nacelles, wings, and weapons, strongly indicates that this particular “refit” operation (on an existing ship) amounted to the entire ship being essentially rebuilt. The Enterprise’s interiors and equipment appear to be entirely new and radically restructured/redesigned. Thus, it would seem obvious that this “refit” in drydock was basically a recycling of the “old” ship and rebuilding it into a new one. This appears to be the Federation’s way of both perpetuating older designs by recycling them into newer generations, and making use of existing vessels as a useful platform to recycle “old” components. It probably also makes it easier when you already have an inhabited ship’s hull to begin a reconstruction operation from the inside-out, section by section. Indeed, if the TMP “refit” operation were typical of how the Federation modernizes old starships, it’s entirely possible that the Constitution-class’ original 12 starships were likely joined by previous-generation starships having been rebuilt to the same or similar spec. This might also explain how the NCC-1017 Constellation could have been of a different (perhaps earlier) starship class/spec that was subsequently “refit” to then-modern (TOS-era) specs.
Back to Reliant. We don’t know when she was built or what she looked like when she was first commissioned. It’s entirely possible Reliant was built in the 2270s, either during or after TMP, and that she looked the way we saw her in TMP2 for her entire lifetime. It’s also entirely possible that Reliant was a TMP-style refit in the 2270’s, having started life configured in an earlier TOS-style spec. What was the history of the Miranda-class? We may never know, but if it paralleled the Constitution-class, the Mirandas could easily have been a link in an ongoing chain of succeeding design specs, constructions and refits.
I favor the original Mirandas paralleling the Constitutions; there may have been a small number of new Mirandas built in the 2240s, with older Miranda-like ships from a previous class rebuilt to the same/similar spec as Constellation probably was.
