A device is already tagged to an organization unit manually and is it possible to remap this same device to a different OU?

Yes. This can be done in one of two ways -

  1. Perform manual mapping to remap the node to the different OU.
  2. Create OU rule under Settings > Organization Units > Organization Unit Mapping Rule with option ‘Include nodes which are mapped to an OU manually’ as shown in below image

Please refer help link Organization Unit for more details.