A project can go back and forth between Revit and Revit LT, being opened, modified, and saved several times.
You can save a Revit LT project using Revit. You can then use Revit to make and save changes, and then open the project in Revit LT again. For more information, see About Working in Revit Projects in Revit LT.
If you use Revit LT to open a workshared Revit project, additional considerations apply. See About Updating Workshared Projects in Revit LT.