I used to subscribe to this idea but I recently was working to develop a very skilled young MC fast trainer over several seasons and was very frustrated to see him deliver form in the 4-5 range. I decided to sell him and then miraculously he started to pull a form of 7'all of a sudden - and he's stayed there. He even delivered an 8 in an important match earlier thus week. Of course, i am overjoyed.
I am still figuring out these patterns as players develop, but I would caution against necessarily assuming a player can't change his spots.