Reference this thread: Re: SNMP Issues
Currently in 10.2, you can only change it if it was an ICMP only node. If it is monitored via WMI or SNMP, the options are grayed out. It would be nice if you change your mind on how you want to poll something that you would have the ability to change it without having to delete and re-add the node, losing any historical data.
I don't know if you guys count voting, but add me to wanting this feature.