Changing out old Consumer Units (also known as Fuse Boxes) in older homes and businesses can have its complications but is often not particularly expensive (see Which Local web site for electrical work typical prices). But it is often well worth the expenditure because in some cases the safety improvement alone is very beneficial. The most likely reason for a consumer unit change is either due a fault in the home electrical system highlighting the need for change or the home owner having a requirement to make changes to home that require electrical installation altered.
The common benefits from a Consumer Unit upgrade include: Considerably improved protection of the users from electric shock, reduced fire risk, less impact on other parts of the electrical systems when a fault does occur, easier and quicker to restore power if there is a fault, easier to modify electrical installation if kitchens or bathrooms are being renovated, replacement or additional components available from manufacturers should changes be required in the future.
My suggestion is that home owners do not wait until you have some other project or reason to change a Consumer Unit, but instead look at it as part of the normal general improvement of a property over the period of ownership. You could spend £500 on a television just for pleasure, so why not £500 on a new Consumer unit to improve the safety of a home costing £100,000s.