Two different things, one button
The app works out which one you mean from the state of the over:
- No balls bowled yet — you picked the wrong name at the end of the last over. The app simply swaps them; nothing is attributed to the wrong bowler.
- Balls already bowled — a mid-over replacement, as when a bowler pulls up injured. The replacement is credited only with the balls they actually bowl, and the over is split correctly between the two in the scorecard.
Changed it by mistake? The app offers to revert a mid-over bowler change, putting the original bowler back with their figures intact.
Why some bowlers are greyed out
A bowler who sent down the previous over cannot bowl the next one — the same rule as the real game. They are shown greyed with a note rather than hidden, so you can see they exist and know why they are unavailable.