MIGRATING SIMULATION MODELS FROM EARLIER OMNeT++ VERSIONS
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From version OMNeT++ 3.x to 4.0, the NED language, the simulation kernel and
other parts of OMNeT++ were completely revised. Hence, simulation models
written for OMNeT++ 3.x cannot be used directly with OMNeT++ 4.0 or later,
and must be ported.

If you have a model written for OMNeT++ 3.x that needs to be ported to the
latest version, we recommend that you do it in two steps: first port it to
OMNeT++ 4.0, 4.6 or some other 4.x version, and then to 5.0 or later.
OMNeT++ 4.x contains a Migration Guide and several migration scripts to
help you with the process. (This 3.x -> 4.0 Migration Guide, along with the
scripts and compatibility features in the simulation kernel, have been
removed in version 5.0.)

Porting models from 4.x to 5.0 is significantly easier. Changes only affect
C++ code (and to a smaller extent, ini files), and are of much smaller scale.
Please refer to doc/API-Changes.txt (also part of the generated API doc) for
details.

Breaking changes within the 4.x and 5.x series are kept to the minimal, and
tend to only affect rarely used or recently introduced feaures. Those changes
are also described in doc/API-Changes.txt.