How to switch from PETG to either PLA/ABS mid print?
You should do a complete calibration for ABS (temperature tower, E steps, flowrate %) before starting the print, then when you change filament remember to apply all the parameters I mentioned.
While I'm not in favour of using the flow rate % to correct the E steps calibration, since you are doing it mid print this may be the easiest way, instead of changing E steps AND flow rate %.
As you said, you need to change temperature too, but both PETG and ABS print well at 235 °C so it may not be needed. PLA works too, but PLA bridges at 235 °C are difficult to get properly.
For sure you won't be able to change other parameters, such as fan speed, printing speed and flowrate for bridges, which are all specific to each filament, but hopefully it will work out anyway, since PETG is trickier than ABS or PLA.
Of course you may have issues with adherence: PETG may not bond well to ABS or PLA (in fact, PETG can be used for support for PLA and viceversa because the bonding is not too strong). You may have a weak print with PLA, so go for ABS.
PETG as support for PLA: Can PETG be used as support material for PLA?
Bridge calibration: