I had to do this myself once.
I found the best solution was to go from HDV1080i60 to HDV1080i50 in Compressor.
Drag the HDV1080i60 video into the batch window, then select HDV1080i50 from the presets. In the ‘Frame Controls’ window, you can set the resize and deinterlace filters to “Fast” as the conversion won’t change either size or interlacing method (make sure that these are both kept the same as your source material).
But you’ll want to set rate conversion to “Best (High Quality Motion Compensated)”. This should give you a very good conversion, but make sure you do a couple tests first – try converting some footage with pans/camera moves or scenes with plenty of movement so you can gauge how effective the rate conversion has been.
Be warned – the process will take a long time! If there is a way that you can just convert your final sequence instead of hours of rushes I would advise you to do that, but if you’re client won’t allow it then you’ll just have to bite the bullet and wait it out.