Okay so in DS9 the Defiant has the Registration of NX-74205. NX meaning it's the first of it's class. Then it's regular registration.
Later in DS9 once the Defiant was destroyed they meant to have the second Defiant have an "A" on the Registry but the cost of changing it in stock footage was too much. Strangely unlike the Enterprise the Registry number changed too and the São Paulo kept it's original Registry number despite there being clear evidence that when you rename a ship to another name it's Registry changes too.
The USS Enterprise-A had another name and registry before it's name changed to what it is now.
So, lets assume we can safely say the second Defiant was "A". The first in DS9 had no letter.
Well, what about the U.S.S. Defiant in that Mirror episode of Enterprise? Wouldn't the DS9 ones be A and B then? Naw...
Multiple times in Star Trek we see future ships sharing the same name as a previous one WITHOUT having a letter. I would guess only the ships with amazing preformance actually get to save their registration and letter. Another possibility is that once a new CLASS is made if it adopts a previous ship's name it restarts on the letters.
An example would be the U.S.S. Prometheus which originally was a NEBULA-class Starship involved in Terraforming. Something may have happened to this ship between the time it was on DS9 and Prometheus appeared in that Voyager episode because I'd find it odd to have two ships of the same name and it's unlikely the first Prometheus was simply retired since it was a relatively newish ship (being a Nebula).
So if the U.S.S. Defiant simply didn't take a new letter because it was the first of an all new class that might explain why the Constitution-class Defiant was ignored. The actual reason being that they didn't know they'd make a Connie have the same name but that doesn't exactly count for lore.
So... Lets assume for SOME reason the DS9 registrations are correct EXCEPT that the second was supposed to have an "A". Lets assume they didn't count the Connie because of either the large gap between ships or because the Defiant represented the start of a new class (or for whatever reason)...
The U.S.S. Defiant in STO is the NCC-75633-C.
How are they on C now? Don't tell me they lost A and B in that same 30 year gap. That's horribly unlucky!
If they counted the Connie I think that would just be too much of a stretch. From some examples in canon it's clear that not every ship that shares the same name of a previous ship gets a letter for whatever reason. I think the Original Defiant should stay letterless.