As far as "the leap" goes, it can be a very difficult transition. I had to do it once with a kara/za guild. The only thing I can say is try to do a mass recruit (don't use trade chat though, trade chat will 90% of the time get you bad raiders) and try to rotate people into the current 10 man setup until you can do 2 10s or 25 mans.
As far as the underperforming warlock. The only thing you can really do is give him advice. Honestly warlock isn't that hard to play, the easiest spec is probably destro. I'd say have him try it out and try to walk through things with him. My guess is he's probably not hit capped, but that's just random speculation. You don't need the best for ICC, but you do need DPS to be higher than the tanks. Frankly if you're on a low pop server with not very many quality players, you're going to have to teach a lot of people how to play.
Another thing I did when I was in this transition was I made another raider rank that was like a "special" raider, and setup separate and objective benchmarks for those players that include exceptional performance (things like for DPS doing at least 5 or 6k dps or something like that, raising the bar as gear becomes more accessible), exceptional attendance, and doing everything they can to min/max their character (all epic gems for example, using the right spec/rotations, always having food and flask, etc.). This special rank also only had a few spots at first to try and engender competition and had perks such as more access to guild bank items and repair money, and were guaranteed raid spots, access to new loot first in some cases, etc. It's not for everyone and it's hard to balance, and a lot of people over focus on it, when really I eventually had to tell people that the special rank was really just a ruse to try and increase overall raid performance. It's just a thought, some people probably might not like it, it worked well for our purposes, and our casual 3 day a week guild ended up clearing all content before WotLK came out back in BC.