I'm in the planning phase of upgrading a 30 breaker panel to a 42 breaker panel (and re-doing all the circuits), and I'm trying to decide if it would be a better move to run conduit from the garage where the panel is up through the attic (about a 30' run), terminate at a large junction box, and then continue the rest of the runs with NM. My reasoning is I don't want to go ripping through all the walls to pull existing wire, so I has planned on cutting the existing circuits up in the attic and either running new NM or re-using the existing NM (assuming it's long enough to reach the location of the junction box).
This isn't commonly done where I'm at, but I'm anticipating adding more circuits in the future, and I think it would be easier to deal with conduit than chase NM all the way through the panel. I'm aware of conduit fill limits and derating; I'd probably need a variety of 1/2", 3/4" and 1" conduit for my needs.
I'm looking for some input to see if others have done this, and whether it's worth it. I'm sure it's personal preference and how much work I'm willing to put into this, but maybe I'm overlooking something.