Played HG:L way back in the day. Noob question. I am downloading atm assuming I must, but thought I'd ask. Thanks.

Time to re-make my Guardian.

Yup you have to download. The Hg: London discs are useless for Hellgate Global.

Thanks for confirming. I assume that most of the game is unchanged besides some of the maps, correct? Obviously the newly developed content is going to be different but I just mean like the classes, skills, etc.

I don't really remember the original so I can't tell you anything about the classes, but there's a bunch of new content and Hellgate Japan will be coming out soon, too.
Also, if you don't want to pay any actual money on the game I suggest you collect as much palladium as possible. You can purchase things such as tickets to enter other areas and other premium items with the in-game marketplace using palladium.