Just cut the roots as they emerge from the bottom of the pot and repot as normal,maybe raise the pots off the ground a little using pot feet or bricks this time.I would water them with fish emulsion after repotting.I doubt you will harm them by doing this.

I repotted my Clivia a few weeks ago, and now have new flowers, so no harm was done in repotting it.

Don"t over water - be sure there is good drainage or they can rot out.

I had two ENORMOUS Clivias given to me by someone leaving Sydney. They were so rootbound but still looked amazing however I wanted to repot as didn"t like the original pots. It was a mammoth effort but I managed to get them out, I spade cut the roots as it was impossible to divide them, then reported them into much larger pots, gave them season and that was 3 years ago and they are bigger and better than ever. You just can"t kill these plants. Ive had some given to me unpotted and have been too busy to plant in the garden, have left them under a tree and months later they are still absolutely fine!!!