The changes are instant when you hide or show a track on your Promo settings screen, so there is no need to resend email invitations.
Hover over the Promos tab at the top of the page and click Manage Promos.
Click to Edit a promo.
Click to Hide or Show specific tracks in the bottom list of songs. Your changes are instantly pushed out to the live promo.
As an example, one of the three tracks is hidden in the Green Day example below. Click Show to make the track visible again.
*** Hidden tracks are not included in a promo's ZIP download. ***
Here is what the promo listener will see: