I have two requirements
All helps are appreciated.
Dynamic PDF can only be created by LiveCycle Designer. You might want to post in one of their forums.
It is possible to build dynamic forms using AcroForms, but it will require some programming.
Just so, it is not possible to change the type of a field programmatically. You would have to delete the field and add a new one with the same coordinates.
However, in your case, it might be sufficient to have a hidden field on top of the signature field, and when the user clicks on the button, that field will become visible. You may or may not remove the signature field in the same step; it might be sufficient to hide it. But that will depend on your workflow.
Hope this can help.