Some things are standardized in every county.  For example "divorce" means a suit to dissolve the marriage.  However, every clerk has their own set of rules and systems for managing current cases.  Chances are that your clerk changed how they "label" your case and "active" and "open" mean the exact same thing.  Another potential cause for the change is that your case was "active", i.e. things were happening... but recently, nothing has happened to further the suit-- thus, it's merely "open."  Which interpretation applies to your situation will depend on your district clerk.  Call them and just ask about the specific meaning.  If it's been several months with no activity, you may want to start wrapping it up because many courts will dismiss inactive cases for "want of prosecution"---meaning that your divorce is lingering with no activity.