| ArionSerpentine |
Rumtum, yes you can modify a construct once it has been created. However the construct can only have up to +50% hitdice added. This is in the rules on constructs in Ultimate Magic.
The debate is not if you can modify it but how much can you modify it by. The rules in the bestiary indicate you can keep adding hitdice but the rules in Ultimate Magic indicate you can only add +50%. I think many of us are figuring it is the difference between 'during creation = unlimited' and 'after creation = limited number of extra HD'.
- Gauss
Rules do exist for modifying a humnuculus after it's creation however it requires a head check in order to successfully do.
The crafter can also add new abilities to an existing homunculus by performing a brief surgical procedure. To do so, the crafter must purchase the necessary reagents and make a successful Heal check (DC equal to the DC of the Craft check necessary to create such a homunculus from scratch). Quote from https://www.d20pfsrd.com/bestiary/monster-listings/constructs/homunculus/
Also gives a listing of possible surgeries