I know its a niche subject but even then just wondering if there were any particular couriers (at least ones originally from 33.6k or 56k eras as far as initial release went firmware-wise) that couldn't be reflashed to be v.92 compliant?
I don't have a great answer. I had a 28.8 model which was upgraded to I believe 56k via a socketed chip swap. Ordered it from US robotics for about $20. My phone line didn't support 56k so I don't fully remember if the upgrade was just to 33.6 or if it was to 56k.
I think the Courier V.Everything had a flashable DSP that could do v.90. I had a Sportster 33.6k modem purchased new that got a free chip swap upgrade to x2. USR later swapped out the whole unit for a v.90 compatible model for free as well. That one surprised me.