You are surely aware that the teflon coupler in the hot end needs to be replaced every 300-400 hours or so. Printing ABS further reduces his life. You could very well be in a situation in which the coupler is deformed so that you can still print ABS at high temperature, but PLA will fail.
Disassemble the hot end, take the teflon coupler out and have a good look at it. Also see if a filament freely slides in it, or if there is friction. Replacing it could solve your underextrusion problems.
Also, when going from ABS to PLA there is the risk that you carry along tiny bits of ABS that will clog the nozzle at the PLA temperature. So another thing to look at is that everything is entirely clean, the feeder, the tube, the hot end. Some recommend, when changing from ABS to PLA, to make a "dummy" 1 hour print in PLA at the ABS temperature, to make sure every possible debris of ABS is entirely cleaned out. Then you go back to the PLA temperature and print your real thing.
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