In the latest PTR build, updated tonight, the Emblem of Valor quartermaster will sell you Emblems of Valor (the 25-man raid, ilvl 213 emblems) for the price of ten Emblems of Heroism (10-man/ilvl 200). This is something we talked about way back last summer, when they introduced the idea of having multiple levels of badges; this way, 10-man raiders can eventually get the 25-man gear, it just takes a very long time. One EoV can not be traded back for ten EoHs, by the way.
I do think that exchange rate might be a bit high. The cheapest items (rings and cloaks) cost 25 EoVs, which would be 250 EoHs if you paid for it entirely that way. That's (just under) 16 weeks of full Naxxramas clears. If you've been clearing Naxxramas for 16 weeks, you're probably pretty bored of it and ready to move on to Ulduar (which will hopefully be released by then), at which point any EoH-EoV trade-up is meaningless, because you're not getting EoHs anymore.