can you print PLA on the .8 nozzle just fine? (I’d suggest clearing the clog- an atomic pull is my go to, maybe followed up with some cleaner filament, then a filament you know well.)

for some comparison, a .8mm nozzle has about four times the crosssection of a .4mm nozzle. (.502 to .125 mm^2), similarly, the volume of a 1mm line printed, assuming you’re using a line width equal to the nozzle and a half your nozzle diameter in layer height, is about 4 times as much, as well. ( .32 mm^3 compared to .08 mm^3.)

in short, given the same printing speed, and the same diameter filament, you’re going to have to extrude 4 times as much filament at any given moment, which means the filament will have to pushed through much faster than a .4mm nozzle, which in turn means that the plastic has about a quarter of the time to heat up before being pushed out of the nozzle tip- and there’s more of it to absorb the heat.

There’s a few things you can try, but probably the only one that would be effective without more or less rebuilding the hotend (and the new-parts-cost associated with that,) is slowing it down.

For the record, if it wasn’t clogged before, it probably is now. since, ultimately, what would be happening is that the .8mm nozzle is causing your printer to print cold.