Can a bowler change during an over?
MCC Law 17 covers an incapacitated or suspended bowler and the rules for the replacement.
An unfinished over has a special case
MCC Law 17 allows another bowler to complete an over if the original bowler is incapacitated or suspended during it. The umpire calls and signals dead ball when the bowler is incapacitated while running up or during the over.
That is different from an ordinary captain's change after a completed over. A partial bowling figure beside a player can reflect this special situation, not necessarily a broken scorecard.
The replacement has restrictions
The replacement must not have bowled any part of the previous over, and cannot bowl any part of the next one. A bowler may change ends, but cannot bowl consecutive overs or parts of each of two consecutive overs in the same innings.
For a hypothetical injured bowler after two valid balls, the remaining valid balls can be delivered by an eligible replacement. The team total does not reset, and the batter's innings does not restart.
Do not infer the reason from statistics
Two bowlers sharing an over does not prove a particular diagnosis or disciplinary story. The official event description supplies the reason. A fractional over figure is a count, not an explanation of what happened.
ScoreZaap's bowling rows and commentary provide different context. Read them together and use the Law for the replacement framework. This guide does not claim an injury occurred in a named fixture or choose which player should complete a real over.