Concerns regarding changing virtual NIC on SQL Server 2008 R2 / Windows Server 2008 R2 I have SQL Server 2008 R2 running on Windows Server 2008 R2 in a virtual environment (ESXI 5.5). Client has been advised to switch the existing E1000 adapter with a VMXNET 3 adapter because they were told it would perform better. 

Anyhow, I've done this several times on member servers as well as domain controllers. As I understand it, SQL Server is more concerned with the network layer so I don't believe this should be an issue. I am not a SQL guru so I figured I would ask here to be safe.
Assuming SQL Server is configured to use a dynamic TCP port via SQL Server Configuration Manager, and the SQL Server Browser service is running and not blocked by a firewall, replacing the network adapter should not change connectivity significantly, within the bounds of the network adapter operational characteristics (i.e. its speed, etc).

If, however, you're using a specific TCP port for SQL Server, you'll want to ensure you document that port number before replacing the network adapter.  Once you install the replacement adapter, and remove the existing adapter, set the new adapter to use the same port via SQL Server Configuration Manager.

This is a snip of the configuration screen: