In 1:8 we are told a new king arose who did not know Yosef.
Rashi brings two opinions as to what that might mean. One, that it was actually a new king and he did not know Yosef. The second, that after Yosef's death the old king simply changed his decrees and acted as if he did not know Yosef.
How could the Pharoah ignore Yosef like that? Even if it was a new king, how could he have been unfamiliar with Yosef - the guy who ran Egypt for so many years and saved it from famine and was responsible for it becoming a great empire? He had to know Yosef!
And if it was the same old king, how could he ignore Yosef? After all that Yosef had done for him, he simply wipes it all away as if it never happened?
Yosef died. He was no longer a political force. He was not someone to be reckoned with any longer. Once Yosef died, the attitude seems to have been that all previous alliances, all deals, all favors owed, etc.. are no longer relevant.
As soon as it was necessary, the (maybe new) Pharoah simply made all his new alliances with others and his new decrees. Nothing from history mattered, as it was time to move on.