ARe you talking physical or Virtual? - If physical vmotion the VMs off - powere doen the ESXi host replace the NICs (making sure they are on the HCL) power up the host - reconfigure the vswitch to use the new NICs - vmotion the VMs back to the host
If virtual - depending on the OS you should be able to do this on the fly and have only a limted break in network connectivity