Transferring a trip to Mobile Pass on the rail planner

I have activated a four day mobile flex pass and completed the initial trip without problems.  But when I return to “My Trips” to assign a trip to a second day’s travel, there is no slider bar available to make the transfer.  I read somewhere that the transfer should be done on the day of travel or even just prior to boarding the train.  Is this true?  If so, that doesn’t offer much time to resolve last minute issues.