When your dentures begin to feel loose, it is typically time for a reline or rebase. Usually you have about a year or more depending on your age, before you begin to experience bone loss. Bone loss will result in a looser denture. A reline allows you to maintain your old dentures while gaining a more comfortable and stable fit.
Relines are not always possible for dentures older than 5 years. In order for a reline to be successful, the reline material needs to properly bond with the old denture. However, as the denture ages, bonding between the old acrylic from the denture and the reline material decreases. If this is the case, then a new denture would be recommended.