I may be mistaken, but I believe STA is being written as a ruleset, not an addon. Rulesets will have their own fields for entries on character sheets, item sheets, NPCs, etc. If you load your previous campaign and switch to the official STA ruleset, it will most likely cause problems unless they (the authors) chose to name fields exactly the same as the author of the ruleset you are using (which I highly doubt). You will most likely have to start fresh with the official STA ruleset, then port over what you can. I wouldn't think it would be that hard since talents and such will probably be drag and dropped onto character sheets.