Well to be fair Piccolo was a villain in Dragon Ball he didn't really switch to hero until Z.

I mean, like half of goku current friends used to be villain at some point in the past:

Piccolo was the last villain of Dragon Ball.
Tien was an antagonist in Dragon Ball as well (villain would probably be pushing it, since he was simply misguided.)
Vegeta… Basically only cemented his turn to full goodness in the late stage of the buu saga.
Even freaking Yamcha was introduced as a bandit fighting Goku.
And I'm not even talking about Frieza and Cell, who are also in Jumpforce.