I like to play on maps I've been updating since 0.12 and over the years I've had mods and removed them. Could there be a "Migration" exception that would replace the modded locos and wagons for the vanilla in the case one decides to uninstall this mod?

Why would you ever want to remove this great mod? :(

Joke aside, you have two options:
A migration script placed outside the mod you want to migrate away, probably a dedicated mod to uninstall another.
The other option is to do it manually from console before removing the mod, but with trains coupling carriages back together is always problematic and can easily end in trains on curves not properly connected.

I know right, but you never know!
I've been doing my half-baked migration scripts over the years as some mods break with updates or I sometimes do my own tweaks.
Was just asking as I rather have a properly done script from the mod creator then my own.

Since you've done migration scripts you should know how to use this:

{
  "entity":
  [
    ["express-locomotive", "locomotive"],
    ["heavy-locomotive", "locomotive"],
    ["nuclear-locomotive", "locomotive"],
    ["express-cargo-wagon", "cargo-wagon"],
    ["heavy-cargo-wagon", "cargo-wagon"],  
    ["express-fluid-wagon", "fluid-wagon"],
    ["heavy-fluid-wagon", "fluid-wagon"]
  ],
  "item":
  [
    ["express-locomotive", "locomotive"],
    ["heavy-locomotive", "locomotive"],
    ["nuclear-locomotive", "locomotive"],
    ["express-cargo-wagon", "cargo-wagon"],
    ["heavy-cargo-wagon", "cargo-wagon"],  
    ["express-fluid-wagon", "fluid-wagon"],
    ["heavy-fluid-wagon", "fluid-wagon"]
  ]
}

Cargo from heavy wagons will go missing and fuel from nuclear locomotives will get stuck with incorrect fuel. No idea how they react should there be spent nuclear fuel in the output when converting back.
I'd recommend letting nuclear locos just vanish.

Many thanks, Optera :D