Usually what happens when people try to do this, is that there is no one transmitting in the new passband when switching to FSK mode. And if there is, the FLDigi 'red lines' tuning indicator is not lined up with the signal being received. Here are a couple of screen shots when using the DATA-U mode to receive, and then the FSK mode to receive. Notice that I moved the FLDigi tuning lines up to 2100-2200 Hz which is where the audio is coming through (actually 2170 according to the digital display just below the waterfall).
?
?
