Can I change the WB 308 Ute pistons to flat ones?

Changing to flat top pistons could lead to issues, regarding timing and other unforeseen problems that would reduce any performance gain, the rest of the engine needs to be equipped to handle the increase in compression. Unless you're rebuilding and doing some high-performance mods to your motor, maybe stick with what you already have. The best pistons are probably the ones that came with your engine.