What can be modded to a kit without breaking it for existing characters?

It seems like any discussion about buffing the underpowered kits probably needs to have this information in mind.
What I'm talking about are things like this: If you were to add the Pelvic Thrust ability, usable once per day per three levels, would a level 8 character start with 0 uses and gain one at level 9? What about stat bonuses based on level, like the Swashbuckler's +1 to AC/hit/damage every 5 levels?
I'm not saying buffs that would break existing characters should not be considered. Though I doubt Overhaul considers it an option.
What I'm talking about are things like this: If you were to add the Pelvic Thrust ability, usable once per day per three levels, would a level 8 character start with 0 uses and gain one at level 9? What about stat bonuses based on level, like the Swashbuckler's +1 to AC/hit/damage every 5 levels?
I'm not saying buffs that would break existing characters should not be considered. Though I doubt Overhaul considers it an option.
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