A change in registers (that hold initialization values) could prevent using an older firmware which doesn't handle the change (back) properly, for example. It depends on what they did in the new firmware probably more so than any desire to restrict users in my experience. They would need to re-write the older firmware which they aren't going to do.
I know that most computers and most firmware based equipment won't let you down rev the firmware version at the consumer level access, so I was really surprised that it did work with the LG BD650. The firmware hacks I've seen for the PS3 to flush Cinavia included a rev number that was one LSD higher than the last installed version, but still lower than the normal OEM uprev increments, so they get around the uprev requirement in the boot strap ROM that way.
But are you implying that if I down rev the firmware on the LG to an older version that it may not really be that version due to some code that may still exist from a newer version?
As a programmer with firmware experience, I would consider that incredibly poor programming practices and would only frustrate their own people trying to figure out what code is actually installed on a particular machine (i.e. if the rev numbers are not corresponding to the actual code). But I guess anything is possible even if it doesn't make sense.
I still don't believe that the newest LG firmware is causing a loading problem because if it was they would have fixed it by now. As far as I can tell, this is the same version they've been posting online for the last 3 months or so. Often the people who answer the service calls, are prone to claim anything without any real knowledge. Especially if the company is contracting out to a call service. It's possible that since you suggested the firmware was a problem, the guy just agreed with you. If it means it delays your sending back an RMA, then that's one of their goals. But I still think it's most likely just a coincidence.
I still think that a firmware improvement that addressed this problem could fix it and hoping for that since I'm out of warranty anyway. But do suggest you send your's back at least 8 weeks before the warranty runs out if it still has problems. Actually, I would recommend sending it back as soon as possible in case you have further problems and they don't want to extend the warranty.
PS I am noticing that with my symptoms, it rejects a BD-R backed up copy within about 4 seconds and with a BD-ROM type original manufactured blu-ray title, it takes about 20 seconds before it rejects it. But once it loads, it plays perfectly; either type of BD. I can pause, rewind, select different chapters and titles with no problems after it finally loads.