Over time we tend to gain more appliances in our homes and businesses. More or larger appliances means more power. More power means thicker cables. Rules and standards change which require switchboards to be wired in a safer manner. New protective devices are invented to keep us safe. Sometimes we outgrow the capability of the switchboard and there is no space for new additions. New technology comes in like Solar systems which require modern switchboards and wiring. Just like we upgrade our computer rarely the switchboard will need an upgrade to reflect modern times. A switchboard upgrade is wired to current standards with modern protective devices, infrastructure and space for new additions in the future.
Older mains cables used to be enough to supply an entire house and now they are only big enough to run a cook top. A mains upgrade is where we install thicker cables that feed the house from the street. This increase the amount of power a house can use.