[bookworm] upgrading.dbk: reword oldrelease upgrade more positive

Minor rewording to be more positive (remove 'not'). Add conditional to the previous upgrade step.

Recognize this change may be subjective and not necessary.

Also not sure if it's worth trying to explain that all earlier releases will require incremental upgrades until they are at 'oldstable'/'oldrelease'; with current wording user will find out in due time as they read previous Release Notes.