"piwo" which Im guessing is the neutar is quite often used as "piwa"
It depends on the so called grammatical case, but it's still neuter. Piwa is the same word as Piwo, but different gram. case. A noun can never change gender, but it can change its ending depening on the grammatical case.
We can take for example the word herbata wich means tea. There are 6 more commonly used grammatical cases.
All these forms are neutral gender. The gender can never change.