I suspect you'll just have to bite the bullet and code yourself a shader.
Really though, it shouldn't be took difficult. You'll need a uniform with a value from 0-1 to control the transition. You'll then need to similarly import the two textures that you want to transition between and then just write yourself a linear interpolation function. Then output the result to your ALBEDO and you're done.
Honestly, it sounds daunting but it's really not that huge a task. Obviously, if you have normals, roughness and so forth then you'll need to repeat the process with that.
Oh, a nice shortcut for you: take your starting texture and convert it to a shader so most of the code will be there for you.
EDIT: silly me, I forgot godot has
mix() built in so you don't even have to code the interpolation. And if you want to get fancy with the transition you can use a Tween cpu side.