Generally agree with the comment from Chris, buying the right thing won't be that expensive online.  Changing those cord ends can be a bit frustrating to do well.  That being said, I don't think 20V vs 19.5V will cause any problems.  Power supply within the PC will be built to step it to the various power levels needed, I don't think anything in a laptop/PC runs off anything near 19/20 volts.  
BUT you should check wattage that the power supplies are capable of.  Possible the Lenovo can't provide as much power as the original all-in-one adapter.  If your old adapter was 140W and you use a 60W adapter, decent chance the voltage would be dropping well below the 19/20V range and that could cause problems with internal power conditioning circuitry.