I don't see how you're going to be able to accomplish this. Your content page is, necessarily, not a full page but just some content controls that link back to the contentplaceholders on the master. If you remove the master then you have content controls that link back to nothing and no <html> <head> <body> <form> tags just a bunch of loose html. One thing you can do is create a second master with the look and feel you want for this particular page when you want it to be a "normal asp.net page" and with contentplaceholders with the same ids as the ones in the main master. Then in Page_Init of this content page you check the condition and if you want the page to be a "normal asp.net page" you set the master to point to this second master page otherwise you leave it pointing to the regular master page.
Another thing you can do (which is not exactly what you are talking about but sort of mimics it) is to put all the content for this particular page into a user control. Then you put the user control in your content page and you also create a second regular asp.net page that uses this same user control. During your Page_Init or Page_Load of the content page you check the condition and based on that either keep loading the page or if you need the "normal asp.net page" you can redirect to the normal page that ha sthe user control in it.
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