Firstly, since you have an ongoing campaign I would take WotC's own advice and not try to convert it to 4E. Make sure the others know that this is not your intent.
Second, you don't actually have the new game in hand and they certainly don't either. Their exclamations of panic are just that - they fear what they do not yet know or understand and are resorting to the bumper-sticker objections they've heard/read others make (yes there are bumper sticker cheers as well). I'd bet money they couldn't really make specific arguments against specific aspects of 4E.
Third, you're not REALLY selling 4E. You're simply interested in trying it yourself - yes? Which means you sort of need the rest of your group to try it as well so that you can ALL find out if you like it or not. That doesn't just mean try it for one session, it means try it for 6 months to a year to UNDERSTAND how and why it does things, and it means being open to accepting that just because it IS different doesn't mean it's BAD for not being 3E. The willingness to give it a meaningful test drive is what you're selling. The 4E rules will then either sell themselves or not.
I'd suggest that what you need to do is start the conversation AFTER the game session. This means that the discussion does not NEED to be boiled down to bumper stickers because the game session itself is about to start and extraneous conversations need to be quickly concluded. Start talking about it when there's TIME to talk about it and those who just don't care to talk about it are free to leave.
They will likely again barrage you with the same bumper stickers. Simply start by saying that you find the changes being made interesting and that YOU are willing to give it a try. When they object, ask them to be specific about their objections:
"They're dumbing it down!"
"What parts are being dumbed down? Does simplification mean stupidity? A major complaint is the complexity of DM prep especially for higher level games - doesn't that CALL for simplifiction?"
"They're making it a WOW clone!"
"In what way? I don't even see how WOW could POSSIBLY measure up to a REAL rpg like D&D. Do you enjoy WOW? What parts of WOW are BAD for D&D? Are those really the parts that 4E is adopting?"
"It's a ploy to make us buy new books!"
"Don't be an ass. It's a new edition. They're not going to GIVE it away. They will have spent 3 years or so in developing it. They're a company, not a commune. Their entire EXISTENCE is based on creating things that they can SELL. 3.5 will no longer have support from WotC and likely almost none from elsewhere. We can of course continue to play 3.5 if we like, but all the NEW stuff will be 4E. I for one would like to TRY it with an open mind and see if we want to EVENTUALLY make the change or stick with 3.5 until we get called stubborn grognards."
KNOW as much as possible about the changes being made because the bumper sticker objections are almost always blown out of proportion. If they object to some change and you are not familiar with that particular aspect - DON'T just reflexively defend it. Simply state that you AREN'T familiar with it. Ask the objector to explain it to YOU and show you examples. Be ready to show THEM examples. Obviously you're not a prosecutor and aren't going to have a notebook full of talking points - but if you remember having read some particular anecdote that will make your point it's more convincing if you can cite where you read it - a blog, a developers post on ENworld, a playtester, one of the sneak-peek books, etc.
