i dont see what the big deal is? Shouldn't it all already be 256 AAC with iTunes Match?
If iTunes Match matches 100% of my music. Which it doesn't. And if I replaced lossless music with lower quality 256 KBit AAC. Which I don't. And if I want to put 256 KBit AAC onto my iPod. Which I might not want to do. This gives the choice of putting 128 KBit, 192 KBit or 256 KBit onto the iPod.
For that, just make sure you have the desired bit rates etc. in "Import Settings", and select all your music, right click, "Create AAC version".
And realise that it _will_ reduce the quality of all your music permanently.