Some months back I had to have an MRI (had to find out if I'd torn a tendon in my knee, which fortunately turned out not to be the case).  I took all my jewelry out for that (it was mostly stainless steel).  It was looking like I was going to need another MRI when I was done there, so I cleverly decided I could leave it out while I waited to find out if I needed the second one.

You know, of course, what happened.  The rest of life kept me busy and I never got 'round to putting the jewelry back in.  This was OK for awhile -- but I've come to realize that I miss having them.  Betcha you've heard that before.

At this point, most of them would need to be repierced, but to my very pleasant surprise, the channels are still open for the ampallang and one tongue piercing.  I wore 12-gauge jewelry in both, but of course at this point the channels have shrunk and the 12-gauge doesn't really want to go through (and I didn't force it).

Thus the question: better to try to slowly stretch these channels again, or just to suck it up and shove the 12-gauge taper through?  (Yeah, ouch, but... it's not like the initial piercing was exactly comfortable.) Have any opinions on this kind of thing?

You should stretch back up gradually to avoid causing trauma and other problems. No shoving!!!
It may require an intermediate jewelry size, but even an old, well-healed piercing can be made to migrate, reject, or infect from that sort of abuse.

Also, I'd suggest having a piercer (or you) try an 18 gauge taper in any of the other piercings you hope are still there. Sometimes the channel just shrinks down, but may be able to be restored to its previous size over time. I've done MANY insertions on piercings people swore were completely closed.

I recently co-authored an article that was printed in Nursing 2008 Magazine about piercing and tattooing myths. It also stated that quality body jewelry should not need to be removed for an MRI if it is not in the area of examination.

And, from The Piercing Bible:
Studies have shown that removing piercing jewelry is generally not necessary for
magnetic resonance imaging (MRI), X-rays, and many other procedures, unless the
piercing is directly in the area of examination or treatment.1 High-quality metal body
jewelry is non-ferromagnetic (nonmagnetic), so it will not react to the MRI equipment.
Beware, however, that cheap body jewelry may indeed be a dangerous problem when
getting an MRI.

Of course, convincing your radiology tech about these things can be a whole other matter.

1. Scott DeBoer et al., “Body Piercing/Tattooing and Trauma Diagnostic Imaging:
Medical    Myths    vs.    Realities,”    Journal    of    Trauma    Nursing    14,    no.    1    (January–March 2007):    35–38.