[old timer]

Edit: See snepp's solution below instead.

I wonder why it's possible to import a schedule right before opening day with historical leagues, but not fictional ones? Is this a bug?

I've added a solution below, but maybe someone else will know of a simpler and/or safer way? If you follow the steps below, you will lose all Spring Training stats for the current season.

You might want to do a test with it or with another league first to be safe. Also, I did this for a league with no minors so it might need adjusting if you have minors.

If you do try this, please make a full backup of your league first, just in case.

1) Go to the league menu then click on Reports & Info, then League Events List.
2) Click the Add Event button and make sure the date is the next day after whatever the current date is. It should do this by default, but just in case...
3) Name the event "Preseason" or whatever you like, then change the "Custom Event" dropdown to "Preseason Starts" and click "Save & Refresh".
4) Next, click the Edit button to the far right of the Opening Day event. Make the opening day at least 3 days from current day and then click Save & Refresh. This will be changed, of course, once you import your schedule.
5) Now advance to the next day. You should now be able to import your new schedule and continue playing.

I just did a test of this (simming to the opening day of the NEXT season) and it worked fine without any problems that I could see, but it's possible to really screw things up with the event editor so I can't promise it won't mess things up.