Usually change has a driving factor which comes mainly in the three, not mutually exclusive, flavors; necessity, efficiency, and profitability. In the first situation, change is unavoidable because the product at the moment is either out of spec, fails to meet a required standard, or has a change in risk(usually an increase) that is too great for governing agencies to accept such as carcinogenic debris from wear of certain materials or previously unknown side effects of particles in the body from “bioresorbable” products or their components. for the second case, improving the cost effectiveness of the product after it has been launched based on new information. This can be changes in the manufacturing process, the density of the material, the amount of coating, and other variables that were initially set high but are now understood to be excessive or unnecessary. In the third category, you can have improvements to existing products to either meet a different customer base, be more appealing to the current base by improving performance, or simply launching an alternative product such as a high end and economy version of a device.
Depending on the nature of the change and how great an impact the change have on the chemistry and performance of the product as well the driving force that initiated the change, only a 510K or supplementing the PMA as PSM34 said. If it is however driven by necessity, the product may end up changing classes.