By the plague of חשך - darkness, the plague brings thick darkness for three days. Then Pharoah calls Moshe and tells him that he has reconsidered and is letting them go to pray in the desert, but only with the animals they need for sacrificing to God. The rest of the cattle needs to stay behind in Egypt (sort of a gaurantee that they will come back). Moshe rejects that and insists that they be allowed out with no restrictions.
In 10:27, one verse later, Hashem hardens Pharoah's and Pharoah no longer wants to let them go, and then throws Moshe and Aharon out of the palace.
This whole discussion between Moshe and Pharoah must have taken place after the plague had already concluded. During the plague itself, we know nobody was able to move, so this could only have taken place after the plague had finished.
This is different than by all the other plagues. In the other plagues, while it was ravaging Egypt, Pharoah pleaded with Moshe to remove it, and he agreed to let them out and then changed his mind after the plague. Or Moshe removed the plague on his own and then brought a new plague, with no discussion between him and Pharoah.
For Pharoah to agree to his demands (in theory) after the plague had concluded is something new.
By the other plagues it was clear he was agreeing to let the Jews go just as a way of gettng the plague cancelled. When the plague was no longer, he was then open to having his heart hardened and change his mind.
This time is different as the plague concludes, then Pharoah agrees to Moshe, then he changes his mind. There is no change from when he agreed to when he changed his mind.
It is clear from the plague of darkness that all semblance of "free will" is gone. While Hashem has been hardening Pharoah's heart, until now He has done it in a way that looks natural. Now he is dropping all appearances. The end-game is near. The level has been ratcheted up a couple of notches. We are in the bottom of the ninth and it is time to get serious.