Bicycle Mechanics - campy cassette cog-9 speed

Currently I have a 53/39 up front. In the back I have 12,13,14,15,16,17,19,21,23.

Going on a ride in late August that has a lot of climbs. I don't think the 23 in the rear is going to cut it for my granny gear. I need another climbing gear.

What would be suggestions on what do go with? 25,26,27 or 28? I can do steep climbs in the 23 but can't sit. I need a sitting gear. Looking at doing 15,000 ft of climbing in 3 days and 200 miles. Those climbs could be rough near the end. Legs might be shot.

Is it possible to then pull the 12 and insert the new cog? Or do I replace one of the other gears?

This is what I'm looking at buying. Can order in any of the sizes. Will one of these work?

www.ebikestop.com/miche_campy_27t_middlefinal_position_cog_9_speed-FW1691.php

That cog should work but you cannot simply pull the 12. The first position cog must have a built-in spacer to work with the lockring. The cheapest fix would be to buy the Miche 27 and pull the largest available single cog. I'm not familiar with your particular cassette so I don't know which single cog you can pull, but the larger the better to minimize the ratio jump caused by the missing cog.
Another option would be to buy a first position 13 (9-speed) pull the 12 and single 13, add the 17 at the back and add a single 18 if, and only if, the 17 and 19 are not joined together on a common carrier.
Here is a source for a 9-speed 13-26: http://www.excelsports.com/new.asp?page=7&major=1&minor=10
That would give you a better climbing gear than your 23 while maintaining a nice close ratio spacing.

Al

Or might this be the best and cost effective way to go?

http://aebike.com/page.cfm?action=details&PageID=30&SKU=FW1575