As you pointed out, these differences are caused by the changes introduced between versions. Unfortunately, there is not way to add a "legacy mode", because most of times we need to rewrite completely large parts of the code, both to introduce new features or improve the performance for some algorithms.
It is not possible to have multiple versions of Forest installed, but you can switch between them easily without reinstalling, only copying some files (and this process may be automatized by a script). The files used by the plugin are described here:http://www.itoosoft.com/forestpack/reference/install.php
Basically, this includes: the plugin (ForestPackPro.dlo), mental ray shader (FShader3.dll, FShader3.mi) and vray DLLs (ForestVRay*.dll)