Simics 6 is not directly compatible with Simics 4.8, since the add-on packages for those versions will not work with Simics 6 and vice versa. However, Simics 6 supports the same kind of installation and multi-user configuration, so upgrading should be very simple.
Simics 6 is API but not ABI compatible with Simics 4.8. This means that user-written modules in projects for those versions have to be recompiled before they can be used with Simics 6. When recompiling, the 4.8 API has to be selected to get access to deprecated parts of the API. This is described in the Simics Migration Guide.
However, add-on packages from Simics 5 will in general work with Simics 6.
Simics 6 releases are binary compatible, so nothing special needs to be done when upgrading. Unless specifically mentioned in the Release Notes, add-on packages should work with any Simics 6 installation. A more detailed description of compatibility issues for user-created modules is available in the Simics Migration Guide.