If the topology of the venue changes, that is, if previously walkable area is closed off or new areas open up, you should collect mapping paths in the new areas to enable positioning in that area, and disable paths in the closed areas. The latter case may require collecting additional paths around the modified area.
The magnetic field fingerprints may change if the venue goes through a major structural change, for example steel structures are being modified or large steel objects like shelves are being relocated. In this case you may need to re-map the areas that have gone through modifications.
Re-mapping could justify itself even without the above breaking changes. For example, a beacon installation can be made to supplement an existing mapping, to improve first fix accuracy.