What more could be said? You risk bricking the device, and having the manufacturer say just that.
Don't upgrade firmware yourself out of warranty, unless you have a proven recovery procedure, that you are prepared to follow.
Unless you are a hacker, and prepared to take extreme measures, such as leveraging an EEPROM programmer to restore the original image..
Have a manufacturer authorized service provider handle the upgrade, and make sure you transfer a risk of failure to them, or don't do it.
If a manufacturer suggests you upgrade... which is unusual, usually they won't provide support to customers out of warranty -- if they do, make sure and get it in writing, and get a promise in writing that they will provide a replacement if the upgrade breaks it.
Or else... pay the manufacturer for out-of-warranty service, and send in the unit.
It's certainly not reasonable to be expecting support, for free, after expiration of the support, though. The manufacturer is not in the wrong refusing to spend money, that wasn't part of the warranty or expected cost of their sale of the hardware to you.