E1000 is an emulated NIC based on the physical E1000 Intel Network Adapter. VMXNET3 is the virtual adapter from VMWare that has a lot less overhead due to the fact that it's not emulating a real life Network Adapter (and therefore no emulation layer). Also VMXNET3 has a 10Gbps rated speed VS the E1000's 1Gbps speed which in large production environments could be a bottleneck (depending on the vm's intended use).
There are a few ways to convert from the E1000 network adapter to VMXNET3, they all require downtime on the virtual NIC and the simplest is to just remove the NIC and add a new VMXNET3 adapter. BUT! You lose the network configuration (and get a new MAC Address) which means you have to reconfigure from scratch (and remembering that your consultant probably "forgot" to provide you with documentation for the configuration before they left!).
The way I prefer is to use PowerCLI to achieve this and to retain the network config. I achieve this by backing up the configuration on the network adapter before I run this using VMUpgradeHelper.bat on the vm's VMWare Tools like this:
1) Before converting the adapter, open CMD and then type this:
"c:\Program Files\VMware\VMware Tools\VMUpgradeHelper.exe" /s
This saves the current config to the registry.
2) Then shutdown your VM
3) Now in PowerCLI, enter this to convert the VM (replace
get-vm <VM Name> | Get-NetworkAdapter | set-networkadapter -type vmxnet3
You will be prompted to confirm with Y or N or A (Yes to All) to convert the adapter. If you have more than one adapter you will be prompted to confirm for each one. Just say N for any you don't want converting!
4) Now, power on your VM, open CMD and enter this to restore your Network Configuration:
"c:\Program Files\VMware\VMware Tools\VMUpgradeHelper.exe" /r
The network config should be restored on your new VMXNET3 NIC's.
You're welcome :)