I’m not entirely sure where that error is coming from, but you should be able to set the extensForm-property to default without problems.
Note that if something goes wrong you can go back to a previously saved version using the local history (Rightclick the form and choose: Replace with… → Local History…).
If you use ExtendForm, and on both forms, you use already the same form elements (before using the ExtenForm) than yes, this will have conflicts!
You are not only extending the form methods, but also both form elements will be ‘kind of’ merged.
Feature Request: it seems that turning ON Extendsform should also warn of any Duplicate / Override Elements !
greg
if something goes wrong you can go back to a previously saved version using the local history (Rightclick the form and choose: Replace with… → Local History…)