I work for Sprint, and if your account is in Ensemble, you will not be affected by canceling the main line.
In Ensemble, each line has the fully priced family plan feature, let's say you have the 149.99 plan for your phones. The 149.99 plan is assigned to EACH line, and Ensemble knows to charge you one time as they all cancel each other out.
In P2K this is not the case, and would probably require you change your plan, because the main line will be assigned the 149.99 plan, and the others will be assigned an attachable plan (such as "149.99 Lines 2-3 Attach", etc) But I have talked to our Sales Support people who can do Add-a-Lines for old Fair and Flexible plans even though those plans are done with, as long as you're already on the plan.
But basically, if you're in Ensemble, there's nothing to worry about.