Sorry about that. We don’t currently have a good way of reconciling tags across different projects.
If we’d thought about it waaaaay back before version 1.0, we could’ve structured things differently to make it more feasible, but as it is tags are identified by a single integer (that happens to be an auto incrementing field in the database) that doesn’t give us any correlation back to the device it belongs to. (e.g., the gadgets are defined to use tag N, instead of tag N on device M)
Extending the page exports with a way to correlate them with tags is on the todo list. That, or a good find/replace interface to help hook them back up (and as a side effect make duplicating a page and switching out tags easy).