In addition to the methods for legacy type mapping described in chapter Legacy Type Mapping there is also the possibility to implement custom legacy type handlers. Those handlers are the most flexible way to do the mapping from old to new types.
The basic interface that has to be implemented is
Fortunately the standard persistence implementation provides the abstract class
one.microstream.persistence.binary.types.BinaryLegacyTypeHandler.AbstractCustom that should be sufficient to start with a custom implementation in most cases.