Like other seized assets they will get repurposed for other investigations. When the feds seize a boat, car, or home they either sell them or use them for operations, undercover and the like.[0]
I imagine the btc will be saved until the case is closed then try to be used for other investigations, I don't think the feds would try converting and selling it.
Why not convert it and sell it? Spending BTC doesn't destroy any evidence, you'll still have a record of the original BTC to trace. I imagine there is some legal procedure to go through before they can dispose of the assets, but I can't imagine them simply shredding the keys.
I imagine the btc will be saved until the case is closed then try to be used for other investigations, I don't think the feds would try converting and selling it.
[0] http://www.justice.gov/jmd/afp/02fundreport/02_2.html