I couldn't find 'classrooms' changing floors but many rooms as a whole have changed floors in Harry Potter due to JK Rowling's mistake
First instance -
At Nearly Headless Nick's Deathday party, Hermione says that Myrtle haunts the bathroom on the first floor. However, she should say on the second floor. Just before they find the writing on the wall, it says "Harry hurtled around the whole of the second floor" and when Hermione is talking to Myrtle, she says "because a cat was attacked just outside your front door on Halloween." Her bathroom is on the second floor, not on the first.
Second instance -
In CoS, on page 119 of the American version, it states, "...Harry was dragging his feet along the SECOND floor corridor to Lockhart's office. He gritted his teeth and knocked." However, on page 141, after Mrs. Norris is found petrified on the second floor, Lockhart urges Professor Dumbledore to use his office to examine her, saying that it is just upstairs. That would mean that his office is on the THIRD floor, not the second.
Third instance -
In Harry Potter and the Goblet of Fire, the entrance to the Headmaster's Office is stated to be on the second floor. However, in Harry Potter and the Half-Blood Prince, the entrance is on the seventh floor. This is easily explained, however, by Hogwarts' magically ever-changing floorplan.