If your Yuuki feels rather on the soft side, than I'm afraid a new one would be firmer than any of the other cups you mention, by a long shot (Diva is a 4 on Melissa's scale, Yuuki currently rates at 7.5).
From the top of my mind, I'd say a large Cuplee or a large Green Donna would be fairly similar. (After I took a look at the chart, a large Naturcup would also fit pretty closely, but I do know the rim is quite a bit firmer than the body, which is very soft, idk if that bothers you.

http://sizecharts.livejournal.com/
http://menstrualcupinfo.wordpress.com/cup-stiffness-comparison-chart/

Edit : According to Melissa's review, the large Yuuki used to be similar to a Ladycup, just somewhat softer in the rim, and firmer in the bottom. I hope any of this helps !

Edited at 2012-07-05 09:41 pm (UTC)

Ok, I must have the old style Yuuki. I just compared my Yuuki and my LadyCup and Melissa's description pretty much nailed it. So, according to those charts you posted, the Yuuki is really stiff now :( That's a serious bummer, I LOVE my Yuuki and I don't like the stiffer cups. I really liked the LadyCup, too, but the rings on the Yuuki make it much easier to grab than the LadyCup. Of course I am more experienced now taking the cup in and out, so maybe I will try my LadyCup again. I need a new one, though, since the one I have has a small tear at one the suction holes (which is disappointing since I only wore it for one cycle). It's the Lotus color, not sure if those had any of the tearing issues. Thanks for the reply.

I also may have to buck up and try to find someone who can ship a JuJu cup to me in the States. It looks size-wise, and softness-wise similar to the old Yuuki, and I need a big cup like that.

Hum, idk really, the Juju is a good 5 mm shorter with a slick stem... if you liked the large Yuuki you must have a fairly high cervix... My Cuplee has those cool grip rings too, it does hold a little less though... Naturcup has the exact same dimensions but somehow it seems to hold a little less as well. if it's a capacity issue, look into Fleurcup and XL Meluna soft.

Edited at 2012-07-05 10:17 pm (UTC)

I do have a very high cervix. I can barely reach the Yuuki sometimes. So I guess the JuJu would not work :/

And I do need the large capacity, I fill up my large Yuuki every two hours on my heavy day.

XL Meluna soft with a full stem then. The shape is different, but the capacity and length are very similar. And it's amongst the softest. Or perhaps, large Si-Bell ? Compare them here : http://menstrualcups.friendhood.net/f52-moderator-s-photo-library

Edited at 2012-07-05 11:00 pm (UTC)

Hi, basically have the same problem as you. I love my older style soft lg yuuki, and have owned many cups...but always use my yuuki because of the capacity, softness, and its easy for me to pop open. I also tried the ladycup (both sizes), and got tired of how slippery they are. I too have a high cervix, and decided to get an xl soft meluna, just in case something happens to my yuuki, but I still find myself using the yuuki. The soft xl meluna can be hard to pop open, and it doesnt fully inflate like the yuuki, so I have to stick my finger all the way up to the rim, and push on it, to see if its open. It's an okay cup...but for someone w/ a high cervix, its hard to tell if its open. Like someone else suggested below, i've been thinking of contacting some sellers to see if they have any old yuukis.

I thought worth mentionning that I LOVE my Cuplee, it's very soft but I don't find it hard to pop open, it's V shaped unlike the Meluna so maybe that's why. The only drawback really, is the capacity is slightly smaller with 25 ml.

Maybe some of the resellers such as Feminine Wear, Enviro Menstruals or even Melissa's store- Zana may have the older style Yuuki in stock.

It's worth a shot just sending an email to them.

I just ended up ordering a new Yuuki. I'll see how it feels when it arrives, but I just love the capacity of my Yuuki, and the length of it (I have a ridiculously high cervix at times), and the fact that it is easy to grasp and pull out when I need to. So I figured I'd just get a new one and see how it is, and I will do a detailed review on the differences once it arrives, since I don't think anyone has done one yet (I haven't found one, anyway). I'm SO bummed that I didn't see the cheapy auctions on ebay back a few months ago :(

I'd say try a Large Fleurcup if the new Yuuki doesn't work for you.
I have both and they're very similar in shape. Plus, Fleurcup seemed to have changed their design to a softer style.