Changing from PLA to ABS/CPE/nylon/any higher temperature filament
Going from PLA to ABS is fairly straightforward. You can just use the 'Change Material' wizard in the printer. Let the ABS come out of the nozzle for a while at the end of the change process, before you finally press the button to end the wizard so you know all of the PLA is out of the nozzle.
Changing from ABS/CPE/nylon/any higher temperature filament to PLA/lower temperature filament
Going the other way can be a bit tougher, because any remnants of ABS will not extrude well at PLA printing temperatures.
In addition to making sure that plenty of ABS flushes out of the nozzle during the change process, you also want to keep the printer set to ABS at the end of the wizard, and then go in to the move material screen (Maintenance -> Advanced -> Move Material) and rotate the dial slowly to flush additional PLA through the nozzle, and get rid of more remnants of ABS. Turn the dial to extrude for about ten seconds, then pause for 5 seconds, so that any ABS in the head can drain back towards the center. Then turn the dial and extrude again for ten seconds, then pause again, and so on. Repeat this three or four times. Only then go into the Material -> Settings menu and pick PLA again.
Alternately, you can set your material to ABS in the Material wizard after inserting PLA and run the first few minutes of a small print (for example, the Ultimaker Robot) in PLA at ABS temperatures. The print isn't going to come out very well, but it should help you get the last of the ABS out of the nozzle. When you're sure there is no more ABS in the nozzle, go ahead and go into the Material -> Settings menu and pick PLA again.
If you do have any remnants of ABS in the head after either method of purging the nozzle, you can try the 'Atomic' cleaning method. If you've been printing in ABS, you'll want to do the Atomic Cleaning with ABS to make sure you grab the bits of ABS filament, since you won't be able to grab them very well at PLA temperatures. The italicized portion of the Atomic cleaning link describes which temperatures should be used for cleaning in ABS.
If you plan to frequently switch between low temperature and high temperature filaments, it may be worth your while to invest in an Olsson block nozzle, if your machine does not already have one, so that you can change out the tips and keep separate nozzle tips for your low temperature and high temperature filaments, and not have to spend as much time cleaning them.
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