Can we change the team sheet after handing it in?

Yes, up to kick-off, under NSL Handbook Rule 16.3, provided:

• the opposing captain, the referee and the match commissioner are told, and
• no player is added who was not on the team sheet you submitted

If a named substitute starts in place of a named starter without this being done, the referee lets the substitute play on, your number of substitutions is not reduced, but the club is charged with misconduct (Rule 16.4).

A player whose name is on the team sheet is treated as having taken part in the match, unless the club removes the name before the match (Rule 16.5). This matters for eligibility, because an ineligible player who is merely on the team sheet can cost you the match (Rule 58.2).