I haven't done it, but have looked at the possibility of doing so for some friends. Making a larger platform for a king is straight-forward, but, as is often the case, the devil is in the details. Just saying 2000-2004 Horizon isn't sufficient to identify those details either, since differing floor plans will present different challenges. For example, 3 of the 4 Horizon floor plans for 2002 had a "north-south" bed (oriented lengthwise rather than across the width of the coach). Those are difficult to install a king because there simply isn't adequate space for a 72" or 76" wide bed and still have room to get around it. The 4th layout for 2002 has the head of the bed in the slideout and low night stands on either side. Cutting out the night stands and widening the platform is not a big challenge, but the wall on one side will be very close, also making it difficult for anybody but a slim person to get into the bed from that side.
Some other types of floor plans that can be problematic include those with ceiling height wardrobe closets next to the bed and those with cabinets or shelving across from the foot of the bed (length is also a concern).
I'm not trying to dissuade you from this kind of upgrade, but wanted to warn you to measure very carefully and remember that you need space to get in/out of the bed and to change linens. And that you also lose some cabinet space.