You can switch from Cast to Human, its just that you'll have to do it manually. Basically there's a value or two that differs from one another, which is what makes your dressing room freeze (Casts use a 'Head' value while Humans use a 'Face' value, if you know what I mean).
Anyway, quick tutorial
You'll need CE for this.
1 - Do a basic Unicode text search for your characters name, once you find it add it to your list, right click and select "Browse Memory Region".
2 - Start a new character and make the appearance of your choice. Before finishing the creation, look over at CE, you'll see something like this..
Just in case you haven't seen my other posts before..
Orange circle = ID
Green = Class Type
Blue = Red Blue and Green values for your hair color
Purple = Your characters proportion (Like height/weight)
3 - So anyway while looking at CE, make a screenshot of the values you see like shown above.
4 - Alt+Backspace or whatever to go back to the character selection and then highlight the character you want to change. From there, select like you're going to the dressing room but don't enter it just yet, like so..
5 - Copy the values of your screenshot exactly as its shown, starting from the orange circle and ending with the purple. When you're done with that, enter the dressing room.
Okay, maybe that wasn't so quick but once you get the hang of it, it will only take you 5min tops for a change.
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Things to keep in mind..
Stats for starters and I'll use your change as an example. If you're switching from CAST to Human, you'll obviously have no MST. All stats (and techs too if you're going back from Human to CAST) get carried over, and your maximum stat will be of your chosen class and not the one you originally had (1237 max atp instead of 1639 for example).
On the bright side.. In addition to your material limit for your HUcast, you'll be able to use more materials reaching a total limit from both classes (Add HUcast material limit with HUnewearls and that's your new max). This also applies for every additional class change too (Meaning you can continue to increase the limit for a larger material usage max).
Example..