I have a pair of 2011 pre Firecrest 303 and a pair of 2012 Firecrest 404.

Both wheels use the 88/188 hubs. Zipp states that only the 2012's are able to be converted to 11speed SRAM/Shimano.

What's the difference between the 2011 188 and 2012 188 that the 2011 can't be converted?

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Different hub and design. The new ones require a re-dish and some work to convert them.

Just sell the ones you got and get the newer ones with 11sp already. Unless you really love them. Then do something like swap the rear 188 for an Alchemy ORC UL Rear 11spd Shimano hub. You won't be disappointed.

^This.

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http://www.zencyclery.com

And this ^^^^

Disregard what is said above about selling your FC404 wheels only to buy a new set designed for 11 speed. Why waste the money and lose money on the sale only to have to buy a new set again? Guys in the industry like Zen will recommend that of course. I say you do some experimenting and make it work with your 10 speed wheels. Read this thread below, especially the posts by elviento. Don't waste money having to sell your 10 speed wheels only to buy in again with 11 speed wheels. That is nonsense.

http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=107427&start=75