funny how cliff wont weigh in on this one, Ive asked it countless times, I assume he thinks rtfrn makes it obvious. My final conclusion on the subject (yes we know there exists hoardes ready to say start from scratch). When you download or update, you do get all the new sounds changes advanced parameters and ALL your sounds automatically change.
However, as Chrisallen states and I think he has it right, your "original" settings may be at odds with the new parameters or even scales of those parameters. So you may have to tweak some things to get even your "new improved" sound back to the way you like. If you HATE your current non tweaked sound, you can do the amp back and forth dance, and RESET all amp model param. to default. However by doing this you may NOW really hate your "old, but updated" preset sound.
Therefore, its back to the old do what your ears prefer. I choose to tweak my old presets which I like without "defaulting" the amp param. For new presets, default is obviously already in play.