Actually, an 'aged up' Rimuru doesn't really seem kosher to me this time, mainly because of how much he wants to respect Shizu's memories. So it just feels wrong for me. (Plus I kinda like how it kinda looks androgyny as well.)
Though there is a fox that 'levels up. It's not really that detailed of a process, and I think the fox was already like an adult when it started.
Before -
https://vignette.wikia.nocookie.net/...20180102160926
After -
https://vignette.wikia.nocookie.net/...20181014175504