Actually, the above paragraph assumed that everyone now living would want to upload their minds into computronium. That assumption was way too optimistic. A significant percentage of the world's population is likely to want to remain in a physical body. This would require us to leave this planet mostly intact. Yes, it would be a terribly inefficient use of matter, from a utilitarian perspective, but it's a big universe. We can afford to leave this planet to the people who want to remain in a physical body. We can even afford to give them a few other planets too, if they really want. It's a big universe, plenty of room for everyone. Just so long as they don't force any other mind to suffer.

You could also almost certainly convert a considerable percentage of the planet's mass to computronium without impacting the planet's ability to support life. A planet isn't a very mass-efficient habitat, and I doubt many people would even notice if most of the core was removed, provided it was replaced with something structurally and electrodynamically equivalent.