Downsizing

In an interesting blue post today, I found that in Cataclysm, while 10s/25s do share the same id - you can only shift down. Basically, a 10 man guild can't shift up. The real thought would be is this going to be on a per boss basis, or once you shift down, are you stuck on 10 man versus 25?

How does this work for end bosses? Take for example Lich King. Sure you have a 25 man raiding guild -and you are consistently getting to and working on the end boss- if you have 10 people who are available and want to put in the extra time, do you let them?

How will this impacting ranking sites? If you share the id, what happens on the achievements/ progression marks? I already know of guilds that will halve runs so that both guilds get credit for the 25 man progression/achievement. In this scenario, I could easily see guilds shifting down to 10 to get the credit.