You could upgrade to 12.0 when you are running an image with not too many features, IP only will run but you will need to add RAM for most others.
What feature set are you running now?
If the upgrade fails: with the 2500 there is a boot load helper, but only as long as you have an IOS running. If you loose that, you are left with the boot monitor which is still ok for newer models, but is pretty basic on a 2500.
ROMMON is located in ROM and is the lowest level and the first loaded operating system (after POST and bootstrap code) residing in a cisco router. It's mainly used for hardware troubleshooting purposes (of course by TAC personel).
Bootloader (sometimes named as RxBoot or MiniIOS) is loaded after rommon during startup and provides a limited subset of full IOS (It provides IP connectivity but can not perform IP routing).
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...