If you read Jason's comments here:
There will be small blocks in the "guard band" and "duplex gap" spaces in the 600 MHz range to squeeze in RF mics. Where these are will likely vary by city, but there will be small spaces - the 600 MHz band isn't completely dead. That, and the fact that there is a 39-month grace period after the auction puts us into 2018 or 2019 - by that point, your equipment may be up for replacement/renewal anyway.
I didn't take the 39 months to be a guarantee but rather as "up to 39 months" will be available but it depends on how quickly the new licensee get on the air. It could be considerably shorter depending on where you are.
Second ... since the National Broadband Plan in place calls for clearing 120 MHz from the TV UHF band does that mean that the 20-34 MHz and another 20 MHz will be targeted in the 500 band by 2021?