For the most part you should be able to switch to 1080p and be good. There used to be a bug that starting in one format, then switching would prevent ALE files from importing. That may have been fixed, or ALE importing is not a need for you. If it is, just create a new project at the correct rate and resolution.
Assuming you have not ingested any footage via import or tape capture, it's fine, except for the caveats Michael mentioned. If you have, that footage is now SD and won't simply switch to higher resolution. It will be upconverted SD. Avid is not resolution independent.