Hello Chris,

is your chassis with a single Sup or it has two Supervisors ?

one point to take care is that there is a chance you get a Sup 720 with catOS.

you can find the migration procedure from CatOS to IOS here

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_install_and_upgrade_technotes_list.html

if you have two supervisors and the replacement one has CatOS to perform the conversion you need to remove the active sup install the new one and perform the procedure above.

if the new Sup has already IOS installed you may need to install the right image to have it form a redundancy pair with the mate (if there is another Sup in the chassis).

All this requires time so you need to ask for a maintanance time window.

Hope to help

Giuseppe