When you import a document that has already been signed, meaning it already contains an electronic signature, you can only:
Add new signers
Drag signature fields wherever you want on the document
But you will not be able to:
Drag initials
Drag read-only text
Drag text input fields
Drag mention fields
Drag checkbox fields
Drag radio button fields
It is not possible to add a signature to an already signed document if the PDF version is less than 1.6
Why? By adding new information to an already signed document, the document is altered, and the previously affixed signature is invalidated.
Yousign does not allow these actions to avoid altering already signed documents.