This post is the product of the findings that modders in the experimental team shared with each others. I won't list them since they may not want to make their involvement in Exp public but I thank each of them.  First the short version : no pre 1.1 plugin (dll) will work without some change. Most of those change are easy, even trivial. But some are complex, especially if your plugin interacts with KSP user interface. This does not list everything but should help