This does not happen often these days, but we are prepared for it. Add-ons can present a lot of different variables. Sometimes we find replacement add-ons to move or convert data (i.e. Power Scripts for Scriptrunner), sometimes we have to manually recreate data, and other times we have to recommend an alternative add-on. In any case, we will give you all your options and provide a recommendation on a case-by-case basis.