I'd make a hard run at him before he hits FA and try and convince him to stay. Sean Lee is getting up there in age, and we all know he's almost a guarantee to miss some games each year.
This is the problem for me.
Say we let Hitchens walk. That would mean Jaylon Smith, another high investment, will be expected to start at MLB next year. I think Jaylon is a great kid and I'm encouraged by what I saw late in the season, but let's be honest, he's still wearing a brace to make it possible for him to lift his foot. I know the hope and expectation is that it will continue to get better, with the possibility he won't need the brace going forward. He was a high selection and the team will want to see a return on their investment.
With Jaylon and Lee slated to be our 2LB crew, the value doesn't really match up with selecting another LB at 19. Your hope is that Jaylon and Lee stay healthy and play excellent, effectively rendering the selection of a round 1 LB to have little impact on the team. However, if either of them goes down, which carries a better possibility here than other teams, then that LB would play a ton.
We already have that guy in Hitchens. He's been an excellent insurance policy who knows the scheme inside and out. Not every player can deliver off the bench when their number is called, but Hitchens is one of those guys.