I think you're stuck until data can be transferred between realms or users, or unless LW is willing to take some time to help out to identify which tag is which in your realm.
Once data transfer is available, I believe you would be able to create a new default realm and use the out-of-the-box race tags to create sample individuals. Name them with their race included (e.g., "Jimmy the Elf") and set the race tag. Export those individuals from the default realm and import them into the realm where you renamed race tags.
Look at the individuals and make note of the race tag associated with them now. If you see Jimmy the Elf's race is now listed as "Vulcan" then you know you renamed the Elf tag to Vulcan.
You can then rename the Vulcan tag back to Elf. Now all your Vulcan individuals show a race of Elf. Create a new race tag for Vulcan, and now go through all the Elf characters and change their race to your (newly-created) Vulcan tag.
Of course, this is all speculation/guesswork on my part, as I have no inside knowledge of Realm Works's workings, but I have done similar things in database cleanup before.
EDIT Addendum: This illustrates why changing the common data can be Bad. If you buy or download a Realms Works dataset from someone else, they will have used that common data, and the terms won't match up. Databases generally don't see "Elf" or "Vulcan" in things like this. Instead they use IDs (either numbers or long strings of letters and numbers called GUIDs) to assign them and refer to them. "Elf" or "Vulcan" is just a label. We can change the label, but we can't change the underlying ID.
When you start sharing data, other datasets will refer to (for example) tag number 15037 in the common data. If you renamed tag 15037 to "Wombat" in your common data, then that is what will show up instead of what the original author intended.