They haven't made any huge changes other than errata. And they incorporate errata into later printings of the books. Everything in the Core Rulebook is still 100% valid if you buy one today. Or don't buy one, as you can find everything in the OGL for free online. And everything Paizo produces that isn't directly tied to their campaign world is OGL.
However, they did release a book recently called "Pathfinder Unchained" which gives some alternate rules to use. Like the 3rd edition Unearthed Arcana, they are optional. The only exception is if you play Pathfinder Society, in which case, you must use the version of the Summoner class (which originally appeared in the Advanced Class Guide) from Pathfinder Unchained. Unchained also introduced an alternate Rogue, Monk, and Barbarian class, as well as new skill systems, which are optional. The new Rogue uses one of the new skill systems, so they are intertwined.
Pathfinder Society play also includes a few other changes you might not expect, but they are relatively minor. Certain feats and archetypes (like 2nd ed AD&D kits, if you are familiar with those) are banned in Society play.
Many of the Adventure Paths introduced new rules systems that add on to the Pathfinder ruleset, but they, too, are optional. Some of them are collected and revised in the Ultimate Campaign hardcover. These include the system of "down time", kingdom building, and a few other subsystems.
So far there has been no "Pathfinder 1.5" or "Pathfinder 2nd Edition" announced. The system has been out for a while, so collecting and becoming familiar with every part of it is a daunting task, but you don't have to use it all, as I'm sure you know.