I need to tell the user that he/she can use the field in a form only if he/she is a pro user. I thought about something like this:
But I'm not very sure if replacing the BROWSE button with PRO is good idea.
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I would not replace 'browse' with 'pro' since pro is not the right label for indicating the action for when the user clicks the button.
Basic and advanced settings
Perhaps you could have a label above the pro settings called 'advanced settings'.
I came across a similar situation yesterday that might help you. See the question here.
The idea is that you only show the basic settings and let the user show/hide advanced settings. Show them that the advanced settings are, well... advanced, so that they are warned.