I have some 2821 rtrs which will be getting an IOS upgrade from "c2800nm-spservicesk9-mz.124-3g.bin" to "c2800nm-adventerprisek9-mz.124-18.bin"
There is not enough space in flash to hold the new IOS and the current IOS.
Is it possible to delete the current IOS from flash and load the new IOS?
As this is your production router and just to be on a safer side, you can also have the IOS loaded on a TFTP and have your router boot via the network TFTP. This will you will have your router up and running incase of something doesnot work while formating the flash and loading the IOS.
Router(config)# boot system tftp IOS filename tftp server ip address - boot from a TFTP server