I'm planning to replace my wellgo 6 degree float to look keo grip 9 degree float. I've searched for cleats replacement guide online but most of them only caters to replacing identical cleats. I'm pretty sure these 2 cleats have some minor differences. How should I go about it? Or should I go to a bike fitter for cleats fitting again? The latter is probably my last resort as it will cost a lot just for cleats fitting.
Wellgo make a variety of aftermarket replacement cleats and pedals for various systems, including Shimano's SPD and Look Keo. They should be (almost) completely compatible with the equivalent system's branded cleats. I've personally used Wellgo Keo-compatible cleats as a replacement for Look-branded cleats, and there were no issues at all.
The way I change cleats is to take a Sharpie and trace around the base (the heel-facing end) of the old cleat before removing it, and then align the new cleat to that line when installing the new ones. It shouldn't require a fitter, since the plastic is as identically shaped as they can make it.
(Just to be clear, trace along where the cleat meets the shoe, as indicated in this picture. This shape is dictated by the shape of the pedal, and any compatible cleat must have this exact shape here.)