“My Lord, I have good news on that score. I have—with difficulty, and after great effort—succeeded in placing an Imperius Curse upon Pius Thicknesse.” Many of those sitting around Yaxley looked impressed; his neighbor, Dolohov, a man with a long, twisted face, clapped him on the back. “It is a start,” said Voldemort. “But Thicknesse is only one man. Scrimgeour must be surrounded by our people before I act. One failed attempt on the Minister’s life will set me back a long way.” (DH, Chapter 1)
“All right, all right, we’ll have time for a cozy catch-up later,” roared Moody over the hubbub, and silence fell in the kitchen. Moody dropped his sacks at his feet and turned to Harry, “As Dedalus probably told you, we had to abandon Plan A. Pius Thicknesse has gone over, which gives us a big problem. (DH, Chapter 4)
Now, the battle of Seven Potters occurred on 27 July, 1997.
OTOH, the Ministry fell on 1 August, 1997 (Bill and Fleur's wedding).
If the Order knew about Thicknesse having "gone over" 4 days in advance, why didn't they warn Minister of Magic Scrimgeour in time so he could de-Imperius Thicknesse and/or take some additional measures to prevent the Ministry from falling 4 days later?
I prefer canon info, if available.