That is completely forbidden. Once you've selected and purchased a Guardian Bakugan, it cannot be changed for any reason whatsoever.
Mylene has always favored Aquos Elico, Abis Omega was merely capable of speech such as variations of Atmos, Verias, and Anchorsaur as well as Hammersaur and Hynoid.
_________________________________________
Sub-Terra Wilda purchased for 3,500 BP.
Ability Card Gun Lock purchased for 1,000 BP.
Total cost is 4,500 BP.