Have there been any rumors of Cisco dropping the 12.4 series of IOS any time soon in favor of the 15.x series?
Not possible. 15.X is very "expensive" in regards to DRAM and Flash requirements. So 12.4 is here to stay.
Which IOS has started going with the Universal image?
ISR G2s (860, 880, 890,1900, 2900, 3900) for routers. Switches are the 2960S, 2360 (I think), 3560E/X, 3750E/X and Sup7. Don't know about the new up-and-coming Sup2T (6500/7600).
How do you migrate a router, say a 2800 ISR, to the universal image. DOn't you need a license key of some kind to unlock the OS feature-sets?
No. You just download the specific feature set.
The only reason I ask is that my 2800s are currently at 256mb RAM w/ 64mb flash, if I need to go to the 15.x IOS, i need 384 RAM w/128 Flash to get all the features we need.
Now here's the funny stuff. For unknown reason, someone in Cisco may have released a "cheap-o" version of the 15.X IOS. In my case, I use the SP Services and it requires 512mb DRAM. I've read (the website has since been corrected) that I can use the 15.1(1)T on 256mb DRAM. I've loaded it for more than 6 months and it works. Now the docu's have changed to 512mb. You can "probably" use 15.1(1)T with 256mb DRAM. You can get a third party CF (maximum size is 2Gb).
By the way, if you are indeed going down this patch and upgrade the IOS, upgrade your bootstrap to 12.4(13r)T6 or 12.4(13r)T11 so you can boot the router using USB.
Hope this helps.
