I read that the best way of removing ABS was to let the temperature at the hot end to drop to around 190deg c then a sharp pull. This worked really well. I am trying to print with PLA but no matter what temperature I drop the hot end to I get left with a length of PLA in the feeder tube. OK I can heat the hot end and poke the excess down with a wire but that is a pain. I think the technique is right but the temperature is wrong. Any help great fully appreciated.
One resource you can use is called the nylon cleaning method. It works by setting nylon filament temperatures, pushing nylon filament into the nozzle until only nylon is extruding, then cooling the hot end to a specific temperature. The page linked suggests a hard yank, but I disagree. Brutality is not a recommended action for 3D printers, in my opinion. When I use the NCM and the hot end reaches the correct cooler temperature, I use pliers and lever them against a suitable surface. The lever action is slower, yet the mechanical advantage is increased, making removal easier.
Some 3D printer users disagree with the expense of nylon, which is, on the surface, excessive. I've found that I am able to see light through the hot-end after using this method, however, so I find the expense justified by a completely clean filament path.
The above linked page also includes the modification of this method for use with the same type of filament to be cleaned, in your case PLA.
Consider that you should be able to use ABS to pull PLA from the nozzle. Heat the nozzle to the lower end of your ABS filament temperature and push or extrude until you get the ABS color. Allow the hot end to cool to the low end of PLA temperatures and reverse the extruder/pull out the filament.
If you use contrasting color filament (for example, white PLA, black ABS) you should be able to see the ABS collecting the other color as you remove it. Eventually, you would have no contrasting color, indicating that the previous filament has been removed.
In line w/ Fred.U's answer, I've been pretty comfortable with the following sequence. Assume a cold start with a filament in the feeder (and cold gunk in the hotend).
1) bring the hotend up to 5 degreesC over your usual extrusion temp for the filament currently in place.
2) If the filament doesn't pull out easily (possible clump at the end), push the filament down and hold it there so the end fully softens/melts. Then remove the filament.
3) Load the new filament desired and push it down until the new material flows freely out of the nozzle.
So with my PLA I heat the nozzle to 180*C, just enough to get it to melt. Push the filament through a little, maybe until a half-inch or so comes through. Once I got that I pull it out as quick as I can. It should not be forceful, meaning no resistance, should just come right out. I have never had a issue doing this and I can switch to a new color or plastic without problems, only having to extrude 4" to get the leftover PLA out of the nozzle.
Hopefully this will help you out a little with this issue.