When new parameters are required, parameterization is focused on the missing terms only. As only new parameters are added, the coverage is increased without damaging any previous coverage.
Any changes made to existing parameters will change calculations results. If existing parameters do not perform well, changes are made for good. The same entries with different parameters are saved with different version numbers in TEAM-FF. Therefore, previous results can be reproduced at any time for validation and comparison.
The most challenging part is when new atom types are introduced. New techniques implemented in DFF ensure the atom types are defined in such as way that previous coverage will not be changed.
A key issue about the extensibility is to identify parameterization molecules. The following screenshot shows missing terms and corresponding fragment (highlighted) in the model: