You can change any Web Form in any way at any time, regardless of whether it's been used to create content items already or not. Alfresco will simply use the new content model from that point forward for all creates / edits of that content type.
Now the big question is "what happens to any existing content items that have the old structure?", and the answer to that is a simple "absolutely nothing". Alfresco will continue to store those content items (in the old structure) until such time as a content contributor edits one of those files - at that time Alfresco will make a "best guess" attempt at retrieving the old data and populating the (updated) Web Form, but from that point on the user will be editing the content using the new structure (so they will need to populate newly added mandatory fields, etc. etc.).
Of course there's nothing stopping you from choosing to do a "big bang" upgrade of all of your content items every time a Web Form changes in some backwards-incompatible way, but unlike some other CMSes we don't force you to do that - it's a choice.
Cheers,
Peter
