Here’s how I switch from ABS to PLA:
(Be careful of course, because the extruder will be hot enough to seriously burn you if you touch it.)
After I printed my last ABS print, I cut the ABS filament off a few inches above the extruder and put the ABS away.
So now I mount the PLA roll, thread it through the guide tube, and then heat up the extruder to ABS temperature.
Just as it’s coming up to temperature, I relieve the tension on the feeder and pull out the ABS stub with long-nose pliers.
Then I insert the PLA, pushing it into the extruder until I see molten plastic flowing from the nozzle.
I quickly tighten the feed tension and I begin extruding 5 - 10 mm at a time via Pronterface.
I watch for the color change, and I continue extruding a bit after the color has fully changed to the PLA filament.
I quickly turn the extruder temperature down to the PLA setting and I keep extruding every few seconds because I don’t want the high temperature to creep up into the tube above the hot end, as this could cause a jam from expanding/melting PLA. I keep extruding until the temperature is close to the PLA setting.
This will be difficult if your ABS and PLA are the same color, so you should do it with different colors first and keep track of how much you had to extrude before the ABS was flushed out. You don’t want to decrease the temperature until you’re sure all the ABS is purged from the extruder.
I’ll let others give you recommendations on the bed coating. I recently removed the PET and my last few prints were PLA on glass with hair spray. It’s sticking OK for some prints, but not quite enough if you have small features that need to stick, so next I’ll give a try with glue stick.
I did like the PET film, except that I kept hacking it up when I pried the prints off, so after 2 sheets I decided to try directly on the glass.