You don't.
Upgrading trough Ferris just makes you lose the -2 enchant level (for example +16 -> +14 weapon). The bloody / dark, augment, appearance, SAs, all remain the same
Upgrading the Bloody/Dark Krishna weapon to Limited (through Mammon), you don't lose anything (enchant level,appearance, SAs, all remain the same)
I think you can only upgrade +10 R99 weapons and up. If you have a +10 Bloody Amaranthine Buster it will be a +8 Bloody Krishna Buster.
If you have a +10 Dark Amaranthine Buster it will be a +8 Dark Krishna Buster.
If you have a regular +10 Blessed Amaranthine Buster it will be a +8 Krishna Buster (not Blessed. Which you have to add the Blessed and then Bloody/Dark enhancement so you can convert it to Limited afterwards).