It is possible. Resoldiering such chips is really no problem when you have the right tools for it. Like a desoldiering heat gun. I have seen a guy at work do it several times. The problem is to find a pin compatible MCU with better specs.
The problem is that the gp32 is basically a Samsung mcu, a bit of ram, and an lcd. If your going to replace the mcu and ram, your basically creating a new system anyway, so may as well start from scratch, which is what Mirko is doing (as the higher-spec mcu's are generally not pin for pin compatible).