It is common to "recycle" an existing airframe by upgrading it in various ways. In fact the customer pressure on Boeing to create the 737 Max (referenced in a sibling comment) was in part driven by the success of Airbus doing the same thing to create the A320neo. The 737 Max is the 9th or 10th variant that Boeing has created of the 737. The FAA has a process for certifying these variants.
The concept of a variant of an existing plane model should not be the focus of concern IMO.
The concept of a variant of an existing plane model should not be the focus of concern IMO.