Yep, did it myself with a GSM Pre2.
Really the only "extra" step is that you need to swap over the old center piece of the phone (if you look at the phone sideways, you can see a small millimeter thick piece of plastic between the two main parts, sortof like the white stuff in an oreo cookie). That piece must be swapped along with the comm board.
It's not terribly difficult, I'd HIGHLY recommend watching the video of the guy doing a pre2 frankenpre conversion in realtime. He does it on a GSM pre2, and it makes the job extremely easy. Here:
FrankenPre Live! The video. | Advanced, Learn, News |webOSroundup
I had that video pulled up on my touchpad while I did the conversion, was easy as pie. Do be careful about the two clips at the top of the keyboard bezel though (you'll know what I mean when you get there, be REALLY careful), also watch out for the volume control board - don't rip it off, be gentle). Also, if you get to doing this and have some issues with the whole software end of it (making a metadoctor), hit me up. I did the full conversion and could "help" you out.
As for your sim card question, the sprint pre is a CDMA board, no sim card used in the pre2 conversion. You -CAN- take the parts from the pre2 however, and install them into your old pre-, making a pre- GSM phone complete with sim card. I did this then used a doctoring process that allows me to use my old pre- as an offline wifi-only device. It's perfect as a simple little mobile web-browser/gps/ipod/media player/gaming device. I haven't ended up using it as much as I thought I would (I mean, I have my pre2 in my pocket), but my point is this whole process can still leave you with 2 functional phones if you wanted it to.