Once a VM is powered on for the first time, a new MAC address is generated for each virtual network adapter, and this MAC address will persist for the VM. unless yo manually change it, or you manually move/copy the VM to another location and answer the question about whether you moved or copied the VM with "I copied it". "I copied it" will generate a new UUID and MAC address for the VM in order to ensure it's unique, "I moved it" will preserve the MAC address.
Unfortunately you cannot just edit an automatically generated MAC address, because the ranges for manually and automatically generated MAC addresses are different and enforced by VMware. If you want to ensure a MAC address remains the same at all times even if you switch virtual network adapters, you may assign a static MAC address to the VM. However, in this case it's your responsibility to keep the MAC addresses unique.
André