If say the Jets and Steelers get into a bidding war, you could extract a couple extra top 75 picks for that pick, especially moving down to 21. Then maybe you could flip 21 for a player.
For moving back only 3 from 13 to 16, at best you get a 2nd, if there are other teams bidding for it. However, more likley a 3rd and a 4th or 5th, since only dropping 3 spots.
I do think its very possible an expensive trade for a vet could come after the draft, after June 1st, since it makes it easier for the trading team to move a high priced unhappy player.
So then it cones down to trading 27' picks when you might need a QB. That's why it's tough balancing the future needs vs all in. It would be great if Stafford can say he fully wants to play at least 2 more years. Takes a little pressure off of getting a QB.