In addition to information provided by previous poster, I do not believe you need to upgrade with the current images you have running, most SPSERVICES k9 images 12.4 versions support netflow.

I also suggest to go over this link as well as the link provided before me.

http://www.cisco.com/en/US/docs/ios/12_4/netflow/configuration/guide/onf_gsg.html#wp1043332

you may verify in features lookup here in feature navigator http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp

another easy way to verify is by typing a question mark at cli without actually entering the command on the routers.

exmaple for this bellow very basic netflow script on a router place a question mark without actual placing command

interface

ip route-cache ?

ip route-cache flow

router(config)#ip flow-export ?

router(config)#ip flow-export version ? <-- should show all the versions supported 1,5,9, etc..

router(config)#ip flow-cache timeout ?

router(config)#ip flow-top-? <--- for top talkers sub-commands

ip flow-export destination

ip flow-export version 5

ip flow-cache timeout active 1

ip flow-cache timeout inactive 15

ip flow-top-talkers

top 10

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(C3845-SPSERVICESK9-M), Version 12.3(11)T10, RELEASE SOFTWARE (fc4) < --- this one should also be able to support netflow

PLS rate any helpful post if it helps

Rgds

Jorge