DCS currently doesn't allow MFD Exports to be changed while the game is running, requiring you to restart to change the export config. As far as im aware this is a limitation of the sim.
For example, the export can't change from MFD 1 to MFD 2 while the sim is running, you'd need to restart the sim. In the case of Harrier as far as im aware, they just run MFD 1 and MFD 2 at the same time with either one just being hidden away through a toggle.