You should be able to simply add key frames to Focal Length (I don't have Blender at hand to check, but I think I've done it recently in one of projects).
Selected one frame, enter value you want at that frame then press
I when hovering mouse over Focal Length value input. Then next frame, enter new value and press
This also lets you use F-Curves editor to specify whatever interpolation of value you want.
Also, I don't think you want to be changing sensor size (though that I would suspect should also support key frames).
Apparently you can't keyframe FOV (I thought it didn't matter whether you operate on FOV or FL), but you can focal length: