I've been changing between PLA and ABS plastics over the Christmas holidays. Every time I flush the extruder from one plastic to another I seem to encounter blockages and general problems, which result at least in me having to dismantle the extruder.
Left to a single plastic, they seem fine. So I've decided to have multiple extruders, one for each plastic type.
So over the holidays, I made myself a new 0.4mm extruder hot end: