I'm made to think of the Overton window, but that doesn't inherently involve change.
I originally posted this as a comment because I didn't think it was a satisfactory answer, but apparently "Overton window" is indeed the term that the asker was trying to remember, so I am adding this as a formal answer.
The linked Wikipedia article says (bold emphasis mine):
The Overton window is the range of policies politically acceptable to the mainstream population at a given time. It is also known as the window of discourse.
The term is named after American policy analyst Joseph Overton, who stated that an idea's political viability depends mainly on whether it falls within this range, rather than on politicians' individual preferences. According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time.
Overton described a spectrum from "more free" to "less free" with regard to government intervention, oriented vertically on an axis, to avoid comparison with the left/right political spectrum. As the spectrum moves or expands, an idea at a given location may become more or less politically acceptable. After Overton's death, his Mackinac Center for Public Policy colleague Joseph Lehman further developed the idea and named it after Overton.
— "Overton window," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Overton_window&oldid=1077856689, copied under CC BY-SA 3.0 licence