Back again. At the end of a completed season during the "offseason," the schedule for the two minor-league teams that I want to change to a later start date (June 20) are blank. That's relevant, because no matter what I do it stays that way. I tried three different things:
1) Nothing. I simmed forward to the preseason and checked the minors' schedules. Nothing there.
2) I then tried re-setting the minors' schedules during the offseason, importing a schedule for the ML team and moving forward into the preseason. Still nothing.
3) Then I tried simming to the start of preseason, resetting the minor league start dates and then importing a schedule for the ML team. Still nothing.
So there seems to be no alternative to getting the starting dates right when you establish the league. It's possible that some funky combination of balanced schedule and/or all-star game stuff (I didn'ty uncheck any of the minors' boxes) would retrieve this situation, but from where I stand it looks kind of hopeless.
So whoever next writes a how-to guide on how to set up a league should be sure to add that one thing you have to do at the beginning is get your starting dates right!