Forgive the probably obvious n00b idiot question. But here goes.
So I ordered a 12-27T Ultegra cassette from probikekit.com to replace my 11-23T 105 cassette. Well it came in a bag completely disassembled. Is this normal? And can I just replace the individual cogs as I wish? I really only want a couple of large cogs to accomodate the hills. I still want the 11t.