You can change it any time. Makes no difference (except that people have already seen fields that are now going to be dependent, for future leads/visits, on multiple fillouts).
Yes just set it to progressive
Yes, you can make the form progressive anytime you want, the basic fundamental of progressive profiling is to use the same form on different landing pages which are used on similar marketing initiatives.
The idea is to collect lead information in stages so by using progressive profiling, choosing form pre-fill as enabled on fields and using the same form on different landing pages will help you get data from leads in stages. Also with this, the data that you already have wouldn't be asked to be filled again by users.
Hope this helps!!
Neeraj, Progressive Profiling does not depend on PreFill.
This is why ProgPro is still able to work on embedded forms.