First check the amount of flash and RAM you have currently. Secondly, check out what feature set you have running currently (eg IP, IP Plus, Firewall, Enterprise etc). Check your router's config and find out whether it runs any special tasks such as QoS, traffic shaping, trunking or not for I was told that these sort of things are the reasons for requiring the "T" train of the IOS. Before downloading the new IOS, check the Release Notes and Config Guide to see whether the new IOS supports the existing extra "tasks" that you're running. Pay special attention to the amount of RAM and flash required for the new IOS (READ THE MEMORY REQUIREMENT LINK). If things look fine then download the new IOS and of course TFTP server. Log on to the router, saved the current IOS (and running config) to TFTP server, erase the flash and upload the new IOS. Reboot and check that everything is working fine and if possible double check again. If things does not work then you'll have to revert back to the old IOS. If you have maintenance contract with Cisco then you can log a call with them and ask them to hellp you out. Good luck.
