The format of the access.xml file is changed from V3 to V4 because of using the Membership provider. When you've converted the site you'll notice that items are not protected anymore
This package installs a dashboard control on the developer section and lets you upload the old V3 access.xml(which can be found in the /data folder) file to migrate its format to v4. Items will be stored in the v4 configuration file /app_data/access.config
When an id can't be found during the process(it is possible that a group was deleted in V3, this didn't update the access.xml file) this will be added to the errorlog. This log will be shown when the process is done.
Once the process is done items will be protected again.