InfoPath Dev

hi everybody, I have an InfoPath form which associated with a form library and 10 items were created in it. I need to update the form that only template change and existing forms don't change (I want to change the value of one field that read the value from another field in the form ). I tried the way that introduces in this article https://support.office.com/en-us/article/publish-an-updated-version-of-a-form-template-084f0b35-fd1e-4938-ad45-2e39784c9d8d, which said this option to do in form option>versioning: To prevent the forms from being updated, click Do nothing (existing forms might not work properly). When this option is selected, users are able to fill out forms based on the updated form template, but they may not be able to open existing forms. If they are able to open existing forms, they won't be able to edit any newly added fields. I did that but when I published the form all the existing forms changed. Would you please tell why this happened?

When you say the existing forms changed, do you mean when they opened you saw your modified template? That would be expected - that if the schema changes don't prevent the old files from opening, they would be displayed in whatever the current template is. However, the underlying data source (XML) won't be updated to the latest schema. If you want your old XML to open with a different version of the template, you can use content types. Take your old template, publish it as a content type, set the library to use content types, set the old template as the default, and relink your existing XML so that they will open with that template. Publish your new template as a new content type, set the new template to the default content type for the library - now new forms use the new template, and old forms use the old template.