-
Don't be dumb
-
that's just idiotic... not to mention that is the reason they gave you the option to create three different characters and play them interchangeably so that you could level them up from start to finish and unlock the character upgrades that suit your play style, if they gave you the option to change your class at level 20 then you would have 0 upgrades that pertain to your chosen class not to mention the armor/weapons are completely different between classes.