The order of steps when unloading filament is really important - changing 1.75mm must be done in a certain order or else you will get your filament jammed in the feeder.
To begin the unloading process, go to menu "Maintenance" > "Advanced" and then select "Preheat Nozzle". Use the dial to set the temperature to 190c-220c and press the dial to enter.
Wait for the value to reach 220c and then go to the option, "Move Material".
Pull the filament guide tube out of the printerhead. The filament should go along with it and you'll notice a large blob at the end.
"Move Material" will allow you to use the dial to extrude/retract the material using the rotary dial - Turn clockwise to advance, counter-clock to reverse. We need to cut the blob away so that it doesn't get stuck in the feeder or filament guide tube. Use a pair scissors or wire cutters to chop that piece off, leaving a nice clean tip.
Now we can reel in the filament until its back in filament guide tube - to fully remove it, select "Change Filament" its safe to do this now because the blob has been removed.
Once fast forward kicks in, make sure your spool doesn't get unwound. Remove the spool from the back and insert the filament guide tube back into the print head.
Ensure the filament guide tube is inserted all the way, passing through outer spring and teflon tube.
Now you can place a new spool on the back and follow the same process for inserting filament.
We been using 1.75mm filament on our Ultimaker 2 for hundreds of hours, kid you not!