If you do small water changes, then the temperature of the water you are introducing should barely change the overall temerature of the tank. For instance, you have a 20 gallon tank at 80 degrees. If you did a 5 gallon change at 60 degrees, I would guess that your temperature after the water change will have dropped to 75 degrees. That is a 5 degree change, so probably too much to do at once. If you did only a 10% change, which is 2 gallons your temperature should drop only to 78, which I would think should be safe especially with a heater going.
I guess if you wanted to do more than a 10% water change, you could always change 10% of it, then wait until the heater gets the temperature back up from 78 to 80 degrees and then do another 10% water change. This will make it so your temperature shouldn't fluxuate more than 2 degrees at a time while using cold water.
This could be wrong, but what seems logical to me. I am pretty new to this, but again, it seems to make sense to me. Someone will let you know if this is wrong I am sure.