Replace content in Wordpress Page after GET from php page

I have a wordpress Page with an html form where i get username and password from my customers.

Then i send to an external php page located in another my server and get the result from inserted credential to check if the user is really allowed to see protected content.

What i am trying to do after login succeded is to replace the current content of wordpress page (the html form) with something else, without change page and without make it external (i want to show to the user the same style of the page).

There is a a sort of dynamic frame or something else that to allow me that?

Thanks very much.