I know I'm walking the line here, so I'll try to keep it on (this) side of proper.
I spent the morning assembling an Astro Spectra Plus from parts from tree different radios I had around the house. The firmware in the assembled radio was old (host/dsp 5) and I tried to flashzap the radio to support some of the additional features I wanted in the radio. Try as I might, I couldn't get the radio into FZ mode. After quite a while searching online, the consensus seems to be that it can't be done. So I gave up and decided I was happy with what I had.
The tuning partition needed to be tweaked, as I now had a frankenradio. After upgrading the tuning partition, I noticed that the beep sequence was off a little when the radio restarted. Just for grins, I tried upgrading the FW again. Much to my surprise, It worked. Tried it with another radio, and that worked as well.
It looks like upgrading the tuning partition leaves the radio in download mode and able to take a firmware upgrade.