Why did they change the registry number of USS Discovery?

Question

In the recent episode S03E06 of Star Trek: Discovery, USS Discovery got lots of upgrades

like detachable warp nacelles held by programmable matter (guess they are going to blow in a future episode) and programmable matter based controls,

but one possibly overlooked major upgrade was its registry number:

(A new letter has been added to USS Discovery's registry number; previously, it was NCC-1031 and now it's NCC-1031-A)

Why did the Federation change the registry number of USS Discovery? Even with the major upgrades, it's the same old ship with Spore Drive unaltered.

Historically, we've seen that a new incremental registry number is assigned to a new ship and that's also when it replaces a destroyed ship of same class. In Star Trek: First Contact movie, when Picard initialized self-destruct sequence of USS Enterprise NCC-1701-E, Picard and Beverly Crusher had this conversation:

Crusher: Think they'll build another one?
Picard: Plenty of letters left in the alphabet.

Talking about Federation's logs according to which USS Discovery was destroyed long ago, the logs can be rectified. Yes, Time Travel was a crime, but Federation was the ultimate authority there. If they trusted USS Discovery and knew that it wasn't involved in temporal war, why couldn't they simply pardon it or change the law?

Why did they change the registry number of USS Discovery?

I want to know answer from both in-universe and out-of-universe perspective.

Answer 1

Registry changes with letters seem to indicate "Generations of Evolution", not necessarily new ships

When we see the USS Voyager in Season 3, Episode 5, "Die Trying", here is what the bridge crew has to say about this:

TILLY: The USS Voyager. (laughs) J. That's... ten generations of evolution?
OWOSEKUN: Eleven. Would love to hear those stories.

This interpretation actually fits the registry changes of the USS Enterprise, which were not only carrying the name to honor the older ships, but can be considered a new evolutionary step for the ship and its respective crew.

However, it's also a good explanation to why the USS Tikhov is referred to on-screen as a ship that has been active for hundreds of years - it has the registration number NCC-1067-M, so we could assume that it has been through 14 "stages of evolution".

It would also explain why the USS Dauntless has already the letter -A attached to it, even though this ship is the first of its class (a prototype, even). With this explanation, they have overhauled the prototype once, justifying a registration change.

At least in my own head, it makes also a lot of more sense to see the USS Relativity with the registration number NCV-474439-G as the eighth iteration of the same ship, rather than as the eighth actual ship with that name.

And then, of course, there is the USS Nash with the registry number NCC-2010-5, and the SS Columbia with NC-5940-1, and the USS Yamato, which changed its registration from NCC-1305-E to NCC-24383 and even NCC-71807. This has never been commented on on-screen, and according to Memory Alpha, at least one of these registration numbers has been a mistake. All these seem to indicate that the system in which registration numbers are assigned are not as iron clad as we, the fans, would like to believe.

In conclusion, after the upgrade which is seen on screen, the Discovery definitely qualifies as "a new generation", which apparently results in a new letter.

However, as pointed out in the comments, that's apparently not a hard-and-fast rule, as the refitted NCC-1701 Enterprise did not receive a letter, and the Defiant-A received the same name, but an additional letter, even though the ship was more or less identical, and certainly not a new "generation".

Answer 2

There is not an in-universe answer or likely will be. At best you might get a comment in tie-in media if some author thinks it's worth commenting on.

A reasonable theory I have seen is the -A is to hide the fact it is a thousand year old ship to the casual registry reader.

Or someone decided enough stuff was changed to justify it.

Out-universe this is an easy way to distinguish the ship model merchandise. This is almost certainly the primary answer.

Discovery writers don't strike me has the type of people that understand that a refit is the same ship and has not previously been cause for a registry change.

Answer 3

Another point apart from the excellent response from Philipp might be in order to avoid detection of the USS Discovery's breach of the temporal accords. See belows video from Star Trek: Discovery - S03E05 - Die Trying.

Transcript (emphasis by me):

Admiral Charles Vance: A quick history lesson. The Federation has spent most of the 30th century fighting a war to uphold the temporal accords. An interstellar treaty outlawing time travel to prevent anyone from the past from changing the future and vice-versa. Your presence here, by definition, is a crime

So, by changing the registration from NCC-1031 to NCC-1031-A, it gives them plausible deniability. They can argue that the USS Discovery is not the same as the one from the 23rd century but a new ship named in honour of an existing ship that vanished in the service of the Federation.