By the way, I got the tubes out successfully. I didn't care for the drywall screw fit, so I used a #6 pan head sheet metal screw. Cut the tip off, and it fit in the collar really well, and threaded into it with effective grip. And they both pulled out all in one piece. I used some heat before pulling to get all the help I could, and it worked out.
I have some new tubes to put in, but noticed they are about 1/16" shorter than the originals. I could knock them deeper into the collar to make up the overall length, but do you know if the depth is that critical?