I found a way to update iMovie
Step 1: follow the advice given in the
post #10 to create a Mavericks VM in VMWare.
Step 2: install VMtools, turn the VM off and then change the display setting: set 3D accelerator ON.
Step 3: follow the same advice as in post
#10 to install the trial or your legitimate DVD/app of iWork and iLife on your VM - do not run any program!
Step 4: install this driver on your VM:
vmsvga2
Step 5: change the resolution to scaled and pick a high one (I used the 2360 - wiggle the window to have vmtools auto adjust the resolution on the host)
Step 6: click update on the iMovie icon in App Store, and your download should continue.
Step 7: In your App Store account you should now find iMovie that you can download on other machines.