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.
[0] http://www.justice.gov/jmd/afp/02fundreport/02_2.html