Can a team replace a player who suffers a concussion?

Yes. If the relevant medical personnel formally notify the Match Referee that a player has concussion, that team can activate a Concussion Substitute for the rest of the match.

• The team's captain (or coach, where the captain cannot practically do so) tells the Match Referee the team wants one and nominates a like-player to the opposing captain for approval. Approval must not be unreasonably withheld. If the opposing captain does not approve, the Match Referee makes the final decision in consultation with Cricket Australia.
• Once approved, the substitute can captain, bat, bowl, keep wicket or field as though they were one of the starting 11.
• The concussed player can take no further part in that match.
• There is no maximum number of Concussion Substitutes in a match.

Apart from this, a player cannot be replaced after the teams are nominated without the opposing captain's consent.