Well, whether you can change the MAC depends on the etherchip in use. Most can be reprogrammed.
Whether the network passes traffic depends on what the hubs, switches and routers expect and are programmed to support. Some can be very restrictive for security reasons.
The general order on an "open" network is to release the IP, change the MAC, then request a new IP via DHCP. If you are using static IPs, then you have to make sure the router's ARP cache is reset to the new MAC.