A successor to first Strive For Power game, currently at alpha stage 路 By Strive4Power
Is there any upgrade or potion that allow us to modify the gender of a character (from male to female/futa or vice versa)?
No
not in vanilla but... you can still use mods (I recommend because I use them myself and I'm satisfied) you would need: Conquest Tweaks by zeltop MGEdits by Magenta_Ivy 馃崉 and SFCRevamp by Chris6
Actually, gender-bender potion exist in Conquest Tweaks mod.