I don't understand this argument and I've heard it a lot. The chargers you have presumably came from devices you purchased, no? They would still work with those devices. I can't tell you how many 30 pin chargers we have around the house with 3 iPhones, 2 iPod Touches, 1 iPad, etc.
Of course, if you're talking about 3rd party chargers (car, etc), I'd be completely wrong and you would be justifiably concerned about having to purchase new chargers to replace these.
edit: and, as I'm typing this, crow216 responds and makes my first question moot.
How cool would it be for Apple to include a 30-pin to whatever (micro USB?) adapter with the new phone just because of this? Doubt they will when they can make more money, but it would be excellent PR for them to head off all the complaints.
It would be even cooler if they included inductive (i.e., non-tethered) charging - thus cutting the final cord for the iPhone. They severed the need for cabled syncing and upgrading; they introduced AirPlay for sound and video transfer and AirPlay mirroring for presentation. The only reason iPhones need a connector now is for charging. It wouldn't surprise me if they're working hard to obviate that final tether. It would also make the transition a lot easier. Instead of a bunch of cables you need an adapter for, you just have a new phone that wouldn't have needed cables anyway. (But if you do want to plug it in, you use a smaller, likely speedier, cable). That's a much better sell from Apple's perspective.
This isn't really true. Changing the screen dimensions means either changing the PPI or changing the screen resolution. They didn't change the screen resolution going to the iPhone 4 (the retina display is still addressed internally as 480x320 unless you do some magic, and most applications don't) and changing the PPI is going to mess with their UX guidelines and result in an inconsistent experience between different devices. The iPhone 4 and 4S aren't going to go away; one or the other will probably become the new bottom-tier free-with-contract phone, and so it's in Apple's interest to keep developers writing apps that are compatible with them.
Apple hasn't given any indication of having a problem with the screen size on their devices. A change would be pretty out-of-character for them.
Actually, the current rumors have the phone's screen increasing in height but not width. This would allow the total screen real-estate to increase without a PPI change. Retina 3.5" displays are currently 960x640; the increase (if the rumors are right) would be to 1136x640, giving you a taller, 4" diagonal screen. How Apple intros the change to devs, if it's real, and uses the extra space, is a good question - but current apps could run fine with a band of extra space either above or below. Or, they could use that strip for notifications, and keep apps the same size... it's anybody's guess.