The 8371 Agnus could handle 512k or 1MB chip, but was not and ECS agnus! You needed to get at least an 8372A to go ECS. I upgraded a number of peoples A500's that contained 8371 Agnus from (512k Chip / 512K Slow mem) to 1meg chip, many years ago, simply by cutting and joining jumper pads. Was very easy.
