Migrate 1 TB of iMovie '11 to iMovie 10.x if migration requires twice the disk space? I have about 1 TB of movies in iMovie '11 (version 9.x) format, and I need to migrate this library to iMovie 10.x now.

This process is described here:
https://support.apple.com/kb/HT202853

However, the page above says that "copies of the original iMovie Projects and iMovie Events folders are left in place". My library is stored on an external drive which is pretty much full.

When I press Update in the iMovie dialog box (Do you want to update your projects and events?), it looks like iMovie is going to copy my old libraries, and therefore require double the space I have on the disk. My little MacBook Air's disk doesn't help.

What do I need to do to migrate this library?
Convert to the internal drive, and as movies are converted, delete from external, and move there. 

If your drives are getting this full, it's time to consider an update. 3TB externals are sub $100.