The extension manager still works. They are going to stop updating it, but at least for this version it still works with the old one.
I have worked all night and made some progress.
If you uninstall and reinstall the Extension Manager... then completely remove and reinstall your extensions, they will install into DW CC 2015.
However there are some UI issues that we have to deal with on our end still. I've managed to install our extensions and activate them. I'm going to need at least a few days to make the necessary changes to get them to look right and function properly now.
I'll update my progress here. WebAssist extension users will still need to hold off updating to CC 2015 until we are able to make the necessary changes.