The most important roots are next to and near the new growth. Leave them be. Delicate and important. Trim the brown dead roots and trim at any break as orchid roots become worthless from a break and down. If a root is bent too much it will crack or break. It would be so much better to watch an experienced grower repot that. Please consider joining a local orchid society, the fastest way to getting answers and learning. Google your county add orchid society and go from there...

I've had to "rehab" plants with almost no roots to speak of. One of the things I learned about repotting cattleyas, is that the plant must be stabilized so it doesn't move in the pot until it establishes itself with new roots in it's new home. As Harvey, said: The new roots just forming are the most important as these will be the ones the plant will use to feed and drink from. So, you want to avoid breaking them, because once they do they stop growing.

I shove short bamboo sticks into the mix, making sure they're firmly in place, no wobble, and then tie the pseudobulbs to them to keep the plant from moving. This goes a long way to protecting the young root leads as they emerge.