At this point there seem to be no documentation for developers how to migrate a Plugin made for Piwik 1.1x to Piwik 2.0.
Problem is mentioned e.g. here:#2391
may become part of
"Provide useful Plugin Developer Documentation"#4122
We will not write documentation to migrate from 1.X to 2.0
Instead we will provide new documentation that will explain the new 2.0 way of building plugins (since a lot has changed)
stay tuned on http://developer.piwik.org/ and #4266