Meh, trading up is no different than trading back or standing pat. It's a crap shoot and also requires the right coaching and scheme fit. Draft a player who is used to playing in a 3/4 and put him in a 4/3, he may never work out.
Take the way the Ravens secondary works, players playing multiple positions on any given play. You can draft a solid corner, but if he can't figure out how to play in your scheme, is a much.
Dejean is one the Ravens wanted, another Hamilton like player, jack of all trades but the Iggles beat them to him. Sure he is good, let us not crown him, shoot the ball was thrown right to him, though he made a nice return and has been a very punt rerurner for them.
As for void years, they are fake years where no money is actually on a contract.
Player has 2 years remaining on his deal, instead of redoing his deal they just add 3 years onto it and take any remaining salary, convert it to a signing bonus and spread it out over the extra 3 years.
Even though he is technically only under contract for 2 years. After the 2 years he is a free agent, if he retires or signed to another team only then will the cap be hit with his current 2 year salary.