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With that series, I believe it's legal to upgrade just one switch, although for redundancy you'll need to upgrade two switches. Whether you buy one or two licenses, you need to insure only the upgraded switch(es) have the IP Services IOS installed. (NB: Although you're legally restricted to only installing IP Services IOS on the switch[es] licensed for it, the stack will [legally, I also believe] run that image across the whole stack.)
