Based on which slot you insert, the nic number might change.
You might loose the uplink for the service console switch.
After adding the NIC, login to console & make sure the service console switch is connected to right NIC.
Like stated above. Take note of your nic config before placing the new nics in. If you have two vswitch's and you just want more nic capacity it should be no problem. Only matters if you have your service console dedicated to a particular nic and then the nic address is changed by adding the new nics. Just power it down and pop them in.