While updating to VSphere, we upgraded the virtual hardware to 7 and the vmware tools to the most up to date version.
After the reboot, VSphere exchanged the old NICs against some new vmxnet3, no problem so far, unless you are using the following scenario und Windows OSes.
1st NIC used for production network.
2nd NIC used for backup purpose or something else.
Due to security issues, we disabled all network sharing and DNS bindings for the 2nd NIC, making it just capable of contacting the backup server and set the 1st NIC to be the primary (advanced network settings).
While exchanging the NICs during hardware upgrade, the order of the NICs got messed up, causing us a lot of DNS problems, applications not being able to contact a server etc.
So while upgrading the hardware version, using more than 1 NIC, check the order of NICs after the upgrade!