Well, the verdict is in:
They don't give back pay. At the time they arranged for me to work at the Glendale location, my manager (for the lab tech position) told me I would be paid the same rate. I guess the manager at Glendale, though, wasn't informed of this, so he didn't know.
It sucks, and seems a stupid rule. Of course they can give back pay, they're just too lazy or too tight to do so. But I'm not ready to make a fuss enough to lose my job [per toga's suggestion :)] over it (or quit). At least I'll be getting some more money each week now.
The "show some love" links are working again! Woo hoo! Let the love pour in.