Bicycle Mechanics - 12-23 and 12-27 cassettes with out re-adjustment possible?

I've got a 12-23 Dura Ace (CS-7700) on an Easton EA90 SL rear wheel. The shifting was perfect to begin with. I put an Ultegra 12-27 (CS-6500) on my Velomax R1 rear wheel. The shifting was immediately rough and I was not able to shift it into the lowest gear (the 27). I tried playing with the rear barrel adjuster to no avail. I put my Easton back on, and got the shifting to about 95% the smoothness it had previously been it set at. I'm at a loss as to how to get these two sets of gears working without having to mess with the alignment.

Is this even possible?

9 speed and 10 speed cassettes are spaced differently between cog centers, thus the shifter moves the chain over a different amount per shift.
You use 9 speed shifters for 9 speed cassettes and 10 speed shifters for 10 speed cassette.

Your chain may have also been too short, if it was properly adjusted for the 12-23, but that's not the major issue.

9 speed and 10 speed cassettes are spaced differently between cog centers, thus the shifter moves the chain over a different amount per shift.
You use 9 speed shifters for 9 speed cassettes and 10 speed shifters for 10 speed cassette.

Your chain may have also been too short, if it was properly adjusted for the 12-23, but that's not the major issue.

Both cassettes are 9 speed (CS-7700 is DA, CS-6500 is Ultegra) so that isn't the issue, since they use the same overall width and same spacing shifting should be the same regardless of cog tooth numbers. The 27 shouldn't really matter over the 23 so long as the chain is properly sized. Also check your B screw adjustment, could be hindering jumping up to the top cog.

If the B adjustment screw doesn't work (90% sure that's it, though) you might need a shim/spacer on the Easton wheels. I think that's the brand I've heard seems to line up cassettes a little differently than most other hubs.

Luckily, my mechanic is in today, so he'll dial in the wheels. I am guessing the spacing is slightly off between the two cassettes, which, if that's the case, he'll know what to do. Hopefully he can teach me his secret to dialing in these things. Seems I just can't get the technique perfected. I'll post back what he does for the benefit of others who run into this problem...

B-screw.

Ok just got back; the problem is not spacing between the cogs, rather the entire cluster is offset about 1mm different than the other. So we just need 1 mm washer to shift the whole thing over. He didn't have the right size in stock, but once he gets it in stock, he said it would fix the problem. He's a pro mechanic and deals with these kinds of issues all the time.