I believe when a Token is activated in our system which gives the Mission Chain, all player results are tracked from that point. It could be that altering from the single to cumulative condition might cause a weird backflow of data? As in doing a reverse check from prior success which could alter mission data after? Lowering the bar to a Co-Op achieveable amount was said to be doable, so glad to have a work-around.
In general, once a mission is started by any player it's hard to interact/alter the mission. Adding paths to completion or additional qualifiers has been shown to be workable (ie: adding Co-Op mode or Scenario modes, or adding "Defended Ribbons" as a trackable condition.)