Repotting a Precious Haemanthus Albiflos

I have a Haemanthus which was my father's - it has been in the same pot since 1985. I managed to grow another from one of it's seeds. The old plant is pot-bound and I wish to repot it safely.
Please could I have advice on what type of medium I should be putting the plants into to ensure they survive?
Thank you.
David H.