It is entirely possible to see a whole different team/set up after the first match (and for every match after that), so long as it is within the rules, after the first posted match is complete?
Why is it necessary to have subs at all for a one day event?
because id rather have the rules in place then think of it on the fly and while it may be a one day event, there are possibilites that some poeple may not be able to make it real life issues etc etc. so better safe then sorry
and while yes it is possible to have a different team each match there is only a 15 minute between matches and the team still has to bewith in the guidelines of the team layout.