If I'm currently running ABS and I want to switch to PLA,

If I’m currently running ABS and I want to switch to PLA, do I need to completely clean out my hot end, or can I just switch filament and chase the ABS out with the PLA? My guess is that won’t work because the high temp necessary to melt the ABS will be too hot for the PLA.

you can just switch. I’ve done it at least twice (didn’t think it would be an issue ) - and nothing bad happened

Just curious - is there a particular reason you’re switching? For functional parts, I’ve been happier with ABS. For artistic, fancy-overhang parts PLA often triumphs (in my experience)

Partially because I can, (I have 1 spool of each and I’ve only ever run ABS, so far) and partially because I don’t have a heated platform, yet, and everyone tells me that PLA works so MUCH better without a heated platform.

When switching filaments, pull out as much as possible of the old filament (pull ABS out at 170-180C, or PLA at 100-110C so that they release from the inside of the nozzle but still have some tensile stength), then push in the new filament and extrude it at the extrusion temperature of the hotter of the two filaments, until you can’t see any of the old color. How much you need to extrude depends on hot end design, relative temperatures, filament types, color, etc., but in most cases running a meter of filament will be enough to purge it (though I’ve seen nozzles with a short hot zone purge fully and less than 20mm, and other times I’ve managed to pull the old filament out entirely, with a piece on the end that is clearly the shape of the inside of the nozzle).

So, when you switch, do you leave the temperature up at ABS temps until the ABS is all out, then drop down to proper PLA temp, or what?

You can run out the ABS with the PLA. You can run PLA at the same temp as ABS. I use a set point of 230C.

that is very true. In my experience - glass w/ hairspray applied every few prints >>>> kapton tape (on a heated bed)

@Jim_Wygralak yes, you need to purge it at the higher of the two temperatures until the old one is gone.