"All bonuses are retroactive when an ability score increases, be they bonuses to damage, to skill ranks, to hit points, to saves, to skill checks... all of them. Skill ranks not being retroactive are a 3.5 convention we specifically removed from the game because it was a weird exception to the rule, and since now there are no exceptions to this rule, there's no need to specifically state that skill ranks are retroactively granted if your Intelligence goes up."
[James Jacobs]
I found this on another site.
RPG Superstar 2015 Top 32, RPG Superstar 2012 Top 32
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When players level and add enough bonuses to their abilities to modify it by +1 do they retroactively apply that bonus to previous levels? IE: If a wizard levels and gains a +1 ability and goes from 17 to 18 do they gain all of the previous levels skills at the +1 in addition to the new +4 bonus they gain by having the +4 modifier? Another instance, if a character gains a +1 to his con, a 15 to 16 (+3) con do they gain the additional hit points for retroactive levels or do they just gain it to that level? I.e., an additional +1? I just need a quick clarification on this. I think they gain the bonus only for their level but I need to know it if retroactively applies to previously gained levels because to my mind they have already gained those levels at they were neither that healthy nor that smart at those levels. An official answer would be great. Thanks.
Have a look in the Glossary of the CRB:
Ability bonuses with a duration greater than 1 day actually increase the relevant ability score after 24 hours. Modify all skills and statistics related to that ability. This might cause you to gain skill points, hit points, and other bonuses.
(Emphasis mine.)