Biggest career turnaround? Obvious example that comes to mind is Warner. 
Undrafted -&gt; Packers -&gt; literally stocking shelves -&gt; AFL -&gt; NFL Europe (which was a thing) -&gt; Rams -&gt; expansion draft fodder -&gt; starter -&gt; 4300+ yrds and 41 tds -&gt; NFL MVP -&gt; Super Bowl Champ -&gt; Super Bowl MVP -&gt; Another Super Bowl -&gt; Another Super Bowl appearance -&gt; plays til he's almost 40 -&gt; Hall of Fame