I have modded mine to print only 1.75. You will use more money to just use that fiament than buying another roll. For my UMO+ to work really fine I had to do:
Change the UMO feeder a bit (modding the plastic with a file, not elegant but works)
Bough like a lot of ptfe 3mm/2mm to insert it on the bowden.
Had to buy ptfe special glue to glue it.
After 2kilos of pla I started to have problems because the pla that is now being pushed sticks and burns on the nozzle and heat barrier/throat.
So I bough a throat and nozzles for 1.75.
Total wasted time 5days.
Money used. Around 150-200€ (I have just ginished installing the second extruder so it everything x2)
Why I did it? Well I have still 400€+ 1.75mm in filaments and they are colorfabb/formfutura not cheap. So I learned a lot of this printer in the process and well it's really fun to thinker with stuff. But no, if you just have 1-2 rolls on 1.75mm it's not worth the effort to adjust something that already works super on 3mm. My 2 cents 
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IRobertI 502
The leftover 1.75mm filament from your old printer can not be used on the Ultimaker2 without modification as it uses 3mm (2.85mm really) filament.
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