Changing type of park ticket lose fastpass?

[Disneydreamer16]

Two people in our party want to change from our 8 day convention ticket to a 10 day park hopper. They realize it will be more money but they want the extra two days in the park and are willing to spend more in order to get it.
My question is related to the fastpasses. We book our fast passes tomorrow morning. If they change their ticket after that point will they lose their fastpasses? I heard that if they change tickets before midnight of the same day then they won't lose anything. Has anyone ever done this before? Should i try and switch them today? I just dont want anything to go wrong tomorrow morning.

 

[disnut8]

Realize that nothing is actually on the ticket itself. It's all on the My Disney Experience account. As long as the new tickets are linked in that account to the FastPasses, all will be good.