I'm not sure about the tweeter issue being on the manufacturing side. I still think they work with Seas to have their tweeters made. They just stopped using ferrofluid in the tweeters and I think they damped the rear chamber differently...but I'm not entirely sure...
The other differences between the previous de capos and the new "i" version is the cabinet as well as connectors and caps.
I had the standard de capos a while back and was also thinking of the upgrade but never did because I didn't feel it was worth the effort or trouble. However I picked up Chuck's de capo-i's here on AC for a song so I could own a pair again as a reference (pun fully intended).
I'd say there is a subtle difference between the de capo and the true "i" version because of the better cabinet control but it's hard to say since I dont' have them side by side. However a friend of mine has the older de capos and we both agree there is something subltly better about the "i" version we just haven't taken the time to do a side by side to figure out what and how much difference there is.
I'd agree with Jonathan on this one to a certain extent. Is a minor improvement worth the time, effort and money you'll put into it? That's ultimately up to you. I do prefer my "i" de capos to my old de capos but they are the redesigned versions, not just the tweeter/cap swap.
If you do decide to go with the "i" upgrade I would recommend a few further upgrades:
-better binding posts such as the Cardas Copper Binding Posts (short) as they are quite a bit better than the stock brass/gold plated versions. Might as well do it all once you have the de capos on the operation table...
-damp the side of the cabinet walls with some premixed ready set concrete patch mix available from your local home depot or lowes. You only need a thin 1/4" layer to damp the sidewalls. You'll need to get the inside of the cabinet slightly damp so the patch mix adheres better. I've used this on a subwoofer and it never shook off, cracked or crumbled.
-Three sorbothane pads; two placed under the front of the de capos and one under the rear. Improvements were immediately heard across the range(cleaner midrange, better highs and bass was still tight with no flabbiness). http://cgi.ebay.com/44-mm-Sorbothane-Isolation-Pads_W0QQitemZ5802149492QQcategoryZ3280QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
">Sorbothane Pads/Pucks couple the speakers to the stands at low frequencies but decouple them from the stands at higher frequencies...Gary at PandaThumbAudio better explained it to me one day when I asked to buy his pandafeet for my de capos but he said they would not work...I then told him I was using these pucks and he gave me the explanation as to why they would work perfectly. Anyway, just try them at the very end of your tweaks so you can hear the difference.