I tried to update iMovie and got the following error.
Update Unavailable with This Apple ID
This update is not available for this Apple ID either because it was bought by a different user or the item was refunded or cancelled.
It is true that I have not purchased it, indeed I don't think I have ever used it. It came with my MacBook Pro when I bought it in 2011. I mainly decided to install to stop the App Store from nagging me about it.
EDIT After a number of calls to Apple Support, and a solution which didn't work I was given a redemption code to install the new version.