Some people have theorised that the place where Takanuva is revived is a machine for reviving Matoran, and that it is why you can see the form of Takua a moment before Takanuva appears. Think of it like this: they put the mask on the Matoran revival machine. The mask's imprint of Takanuva makes it possible to revive him. Takua appears. Takua is immediately transformed back to Takanuva due to the presence of the Avohkii.
However, as I understand it, Matoran transforming into Toa is a pretty dramatic and nearly irreversible process, changing the actual mechanical and biological components of the character physically. So maybe this theory does not make sense.