I highly doubt a lot of people will leave with the new client. I am sure a
small handful will leave and squawk very loudly as they do, But the fast overwhelming majority of people I communicate with online and in the real world are looking forward to the upgrade and can't wait for it to get here.
the things that make Ultima Online a great game will still be in the game crafting, housing, etc... they are not throwing out the game and implementing something completely different and calling it Ultima they are just upgrading the graphics and interface. (witch are essentially the same as when I started almost 10 years ago on my 486dx2 computer)
just about everyone I talk to about the graphics upgrade all say the same thing ... "about time.." "
wow it looks really good I can't wait.." and so on.
the reason people play Ultima has never been about the graphics it has been about two things 1) content and diversity of game play and 2) the isometric view (3/4 top down) and these are not changing in any way with the new client. A house doesn't change because it gets a new coat of paint its still the same house.
so myself and a lot of others will still be here even after the upgrade. it is a great game and will continue to be a great game.