If you need to move ALL uploads from one gallery to another, without having to manually select them, follow the steps below:
From the host account, go to the main event view - where you'll see your event analytics. If you're in a gallery/album (viewing photos & videos), click the back arrow next to the gallery name to exit into the main event view.
Click the Menu button at the top right (purple circle with 3 white lines) to open the "Event Menu"
Select "Merge Galleries" from the menu list
Select the Source Gallery (where you want to move uploads FROM), and the Destination Gallery (where you want to move the uploads TO). If you want to delete the Source Gallery after merging, switch on the "Delete source gallery after merging".
Click the "Merge" button