Rebranding is a deliberate change to how a brand looks and communicates: from refreshing the logo and identity to a new name and strategy. Companies do it when the existing brand stops supporting their goals.
Rebranding comes in degrees: from a facelift (refreshing the mark and typography while keeping recognition) to a full change including the name and positioning. Good reasons include a shift in offer or audience, a merger, international expansion, or a brand that visually lags behind the real quality of the company.
The design itself is usually the smaller part of the cost. The real bill comes with rollout: website, sales materials, signage, packaging, company profiles, document templates. That's why a rebrand is planned like an operational project, with a list of touchpoints and an order for replacing them.
Warning sign: if the only argument for a rebrand is "we've grown tired of the logo", hold off. Recognition takes years to build and is a real asset; sometimes tightening the rules for using the mark works better than replacing it.