WhichAmiga may be wrong, but a 8367 Agnus is a DIL type and would not be upgradable.
If there's a PLCC type in place (should be with the composite output), you can very easily swap that with an ECS type, change 2 jumpers and you've got 1 MB. For 2 MB it's the Mega-chip route.
@shaf
There are (very) different rev 4 mainboards. 4.0 usually is the A1000 chipset German 2000, the A500 chipset American 4.0 rev is extremely rare. 4.1 onwards is always A500 chipset (PLCC Agnus) & Buster.