1. Stop your application via the Synology package center UI.
2. Login to Synology using SSH and elevate to root.
3. Use “ls” against /VolumeX/@appstore to find your package folder name
4. Use “mv” to move the data between the old and new volume, you may need to create the @appstore folder first.
5. Remove the symlink from the current volume and create the new symlink to the new volume.
6. Stop and start your package in the Synology Package Centre.

Below example commands;

sudo mkdir /volume2/@appstore
sudo mv "/volume1/@appstore/Plex Media Server" "/Volume2/@appstore"
sudo rm "/var/packages/Plex Media Server/target"
sudo ln -s "/volume2/@appstore/Plex Media Server" "/var/packages/Plex Media Server/target"