It's a bit confusing...there are all of these timers and restrictions to changing campaigns, but you can do it without spending those AP.

For a home campaign, I think you need to allow the current campaign to end, then do NOT return with that character in the new scoring session. I think this is when the system will allow you to change via the campaign menu without the cost. It's also possible the cost will go away if you don't go into the campaign for 4+ days...but this might only apply to your guest campaign.