clarification: images vs upgrade

[arne anka]

> If I understand it correctly, no. If you upgrade, you get new versions
> of each installed package. If you flash, you lose all software you
> installed ”over“ the basic system.

let's forget about additional packages or changed settings for the moment.
a new image should contain newer versions of packages than the images  
before, shouldn't it?
so in respect to the packages installed by default flashing a new image  
should be equivalent to doing update && upgrade, right?

i still try to figure out whether i need to flash or update && upgrade  
does the same for me.