I’m creating a Blood Sorcery Sorcerer and trying to replace all Charisma ability checks with Constitution, or outright replace CHA with CON. How can I accomplish either of these with the tools at my disposal?

replacing ability checks is actually easy, though you'll have to do it manually. On the character sheet by the skill section, you can click it to open it up, press 'customize' and it will let you manually override skills; the governing stat, whether you have proficiency, expertise or nothing at all. You can also make full customized skills along with the governing stats etc, I usually do this for tools that I actually need (thieve's tool, etc)

Regarding to changing the spellcaster modifier.. currently, I don't think you can though they said it may be an option during the homebrew revamp

When you create a subclass there is an option at the bottom, below description, "Spell Casting Ability" - change this to the one you want. Now all spell casting uses that ability instead of Cha. 

There is no option to do this for skills, AFAIK, so the person taking the subclass would have to manually change the skills on their sheet.

Quote from Cyb3rM1nd >>

When you create a subclass there is an option at the bottom, below description, "Spell Casting Ability" - change this to the one you want. Now all spell casting uses that ability instead of Cha. 

There is no option to do this for skills, AFAIK, so the person taking the subclass would have to manually change the skills on their sheet.

 That's actually great. Would this work for an already spellcasting class too or only like arcane tricksters and eldritch knights

Quote from Noxx89 >>

Quote from Cyb3rM1nd >>

When you create a subclass there is an option at the bottom, below description, "Spell Casting Ability" - change this to the one you want. Now all spell casting uses that ability instead of Cha. 

There is no option to do this for skills, AFAIK, so the person taking the subclass would have to manually change the skills on their sheet.

 That's actually great. Would this work for an already spellcasting class too or only like arcane tricksters and eldritch knights

 
I tested with a Sorcerer subclass and it worked.