Switching routes from 1 plane to another

Yes you can. There are two ways actually.

The first method is finding the route you want to renew and go into it. By default, you will end up on the "Market Research" tab. The tab you want is "Flight list". When you've switched tabs, you select the round trips you want to renew on by clicking their individual checkboxes. (or you could click the checkbox that is on the top of the list to select everything). Now, you go to the bottom of the page where there will be a selection of what you want to do with the selected round trips. To replace an aircraft, select the aircraft type you want to replace the existing aircraft type with and click "Change Aircraft". Now, a yellow box will appear on top of the list. There, you will click the checkbox of what aircraft you want the old one to be replaced with and you should hopefully figure out everything else.

If the yellow box shows no aircraft, check the runway length. If that's not the problem, lower the amount of flights you want to move to the aircraft.

The other way involves finding the aircraft you want to replace. You can find all of your aircraft by clicking the "Aircraft" tab on the left side on the screen. Now, you find the aircraft you want to replace and repeat the process stated above, starting from switching over to "Flight list".