SC2Mapster

I'm trying to figure out the simplest way to give a unit 2 models, that can be accessed via an ingame command that simply switches between them.

A follow up question would be how to make the second model's scaling/proportions match the first model.

Teach me wise ones! 

It's a custom command, you type something like "-alt" and it causes the model to switch. Entering -alt again, causes them to switch back. That's the idea at least. 

When trigger generating a model swap actor message the variation parameter must be 0 and not its default of -1 or it will throw an error.

Here is an example trigger demonstrating a command to swap selected units to any model, entered by the argument of the command. Eg all selected units will be model swapped to the marine model with the command "-swap Marine".

Model Swap Test
    Events
        Game - Player Any Player types a chat message containing "-swap ", matching Partially
    Local Variables
        chat message = (Entered chat string) <String>
        arg1 = (Word 2 of chat message) <String>
        arg1 parsed = No Game Link <Game Link - Model>
        u = No Unit <Unit>
    Conditions
        (String((Word 1 of chat message)) as Lower Case) == "-swap"
    Actions
        UI - Display "Model Swap Test" for (All players) to Subtitle area
        Variable - Set arg1 parsed = (Game Link(arg1))
        General - If (Conditions) then do (Actions) else do (Actions)
            If
                (Catalog Models entry arg1 parsed is valid) == True
            Then
                UI - Display (Text((String(arg1 parsed)))) for (All players) to Subtitle area
                Unit Group - For each unit u in (Selected units for player (Triggering player)) do (Actions)
                    Actions
                        Actor - Send actor message (ModelSwap arg1 parsed 0) to main actor of unit u
            Else
                UI - Display "Invalid Type" for (All players) to Subtitle area

This is perfect! I got it to work. Thank you guys!