Nothing is certain yet, but the most logical way this extension will work is as follows: Prior to the reset date, you can click the Extend Raid Lock button, which will ensure that your raid does not reset as it normally would. If you remove the extension later, the raid will not immediately reset. Rather, it will reset the next time that it normally would for your server.
If that's how it works, it will balance out the advantage of spending more time on later bosses with the disadvantage of obtaining less loot in a given week. Extending or not extending the lockout will be a tough choice for raid leaders. In many ways, this decision will define your guild. Is loot your priority, or do you want to progress through the zone? Contrary to some players' opinions, this is not a hardcore-vs.-casual issue.
Many hardcore guilds want that extra loot every week. Giving it up is a risk. They may hit a gear check they aren't ready for and will find themselves a week or two behind where they should be. Casual guilds want loot, too, obviously. They run the risk that by spending an entire week wiping to bosses they haven't downed yet, their players may get too frustrated.
The crux of the problem is that different players enjoy different things. Some prefer to relax and farm the bosses that have been conquered. Others prefer the challenge of taking on new bosses. In a typical raid week, your players get to do both (even if the "new" bosses are just the same bosses on hard mode). If you extend the lockout, only the players who enjoy a challenge will be happy about it.
Let's not forget the loot issue, either. Players who are ahead of the loot curve won't mind if you extend the lockout and push forward. Other players are behind the curve and may be looking for specific drops from earlier bosses. They won't be thrilled to hear that they'll have to wait a week for another chance at The Lifebinder.
When you're first starting a new tier, the decision is fairly easy: You'll have to gear up. And when you've farmed a place dry for many weeks, the decision is also fairly easy: Keep that lockout and work on hard modes or achievements.
But when you're in the sweet spot between farming a zone and trying to master it, you could be in for some real drama over this issue. My guild is currently in this spot for Ulduar. We've killed every boss except Yogg. We've also done some partial hard modes and achievements on our runs so far, but we really should have killed Yogg by now. Summer has a way of slowing our progression, and this year has been no exception. So, if 3.2 went live today, would we extend our lockout in order to kill Yogg?
My vote would be yes. I'm not one to be satisfied with farming when there are unkilled bosses floating around. I want to see Yogg go down. But I know that some of my raiders would be against that decision.
The key to preventing drama over the lockout issue will be good communication. When you have a fresh instance, set a goal for your raid for the week. Then make sure everyone knows that if you don't succeed that week, you'll extend the lockout. That will serve as extra motivation for those who don't want the lockout extended. Just make sure it's a realistic and significant goal, or you'll hurt morale.
It's not clear yet if you can extend lockouts indefinitely. If you can, make sure to put a cap on how many times you'll extend it. For example, if my guild were starting a fresh Ulduar today with this option, my plan would be this: "If we don't kill Yogg this week, I'll extend it for another week and we'll focus on Yogg. If we somehow don't get him next week, then I'll let the raid reset so we can farm bosses again."
The great thing about a zone like Ulduar is that optional bosses can be skipped as a concession. By going right for Yogg, and skipping Razorscale, Ignis, and the Assembly of Iron, you can give yourself more time to achieve other goals. That way, you can decrease the chance that you'll have to extend the lockout. If you do have to extend it, you can kill those other bosses the next week to give your more loot-oriented players something to look forward to. If you achieve your goal early in the extended week and have some extra time, you can spice things up by going for achievements like Shattered or Can't Do That While Stunned.
Communicate your plan prior to a week's first pull, so no one is surprised or dismayed by what bosses you skip or your decision to extend the lock. People may still grumble, but at least they can't say they didn't know your intentions.
Whether your guild wants to face Algalon or simply see new bosses rather than farm the same ones week after week, you'll soon have the flexibility to achieve these goals at your own pace. And that's a good thing for everyone!