Wrong. Although it's not very common I've seen it happen. The Rams were 4-12 and last in their division in 1998. A year later, they were hoisting the Super Bowl trophy in Atlanta.
The Carolina Panthers finished 1-15 in 2001. Two years later, they were a hair's breath of beating the Pats in Super Bowl XXXVIII
Hell, both the Jacksonville Jaguars and the Carolina Panthers were expansion teams in 1995. A year later, both were playing for their respective conference titles.
None of those are examples of "worst to first overnight"
If you had said something about the 2007 Celtics, I might have believed you.
However, 2 seasons is hardly "overnight" and playing for conference titles and losing is not "first." The Rams were last in their division, but not last in the league. The Eagles, Colts, and Bengals were all 3-13 that season.