Queue the first ship for production. Look at the name assigned to the ship in the production queue. Name the design that you just queued up the "<name> Class".
If you're not familiar enough with the historical ships to know what kind of ship it is from the name, or have any expectations about the kinds of names for a given class, then are you really getting any immersion from using those names? "Immersion" seems to depend on at least as much knowledge as being able to name the ships yourself. Kind of have to know what you're missing to miss something.
I often tend to avoid that bit of immersion in favor of practicality and name my classes something like "CL 40", "ASW 40", "DE 36", "BB 44", etc. Easier to find the right one later in the war.
The most common case I have for two classes of the same type is with destroyers -- ASW or fleet combatant designs. Upgrades for tech just get a suffix ("ASW 40 B" or "ASW 40 II" or "Mk II" depending on my whim). I do usually keep the historical name lists, but occasionally name some capital ships.
Never modify the base hull you research into a ship. Leave that with an awkward and annoying name so that you don't forget that you need to spin off a variant for your actual first ship class. Otherwise, you don't have that empty hull to work with for a later variant, and have to spend IC ripping out stuff to get back to the empty hull.
