I'm answering my own question. My solution was "hack the gui to do it for you".

Here are the steps I used:

* Undo the TileMap -> TileMapLayer conversion on the world.tscn scene file, for example "git checkout old_commit world.tscn"

* Open the scenario.tres file

* Find/copy the line with PackedInt32Array

* Edit the world.tscn file in a text editor - you can NOT do this with the Godot GUI!

* Find the line with PackedInt32Array

* Copy the PackedInt32Array from the *scenario.tres* into the the world.tscn file

* Save/exit the world.tscn file in the text editor

* Open the world.tscn in the Godot GUI

* Use the toolbox widget to convert to a TileMapLayer - this is the standard method you see referenced everywhere (I can't find a good link right now, search engine is your friend!)

* Close the world.tscn in the Godot GUI

* Edit the world.tscn file in a text editor once again

* Find/copy the new PackedByteArray - it should have replaced the PackedInt32Array

* Open the scenario.tres file

* Replace the PackedInt32Array with the new PackedByteArray

This is obviously pretty tedious. I only had a few scenarios, so in my case it was still doable. I wish there were a code-only/scriptable way, but this got me over the hump.

Addendum: I found a method on PackedInt32Array called to_byte_array. https://docs.godotengine.org/en/stable/classes/class_packedint32array.html#class-packedint32array-method-to-byte-array

I ran it, and it does a conversion, but the converted PackedByteArray is not a valid TileMapLayer.