Q1. Why do we need to setup a new DB?
New DB not required.if you had backup of existing DB, Because TN,IS, MWS,… scripts vary alot between older version to newer versions, you know webmethods have alot of dependency on transactional database esp in case of TN
So make it easier without losing existing partner information & other TN Assests, We run migration scripts on existing database
Q2.If you are setting up a new DB, how do you move the existing DB assets to the new DB?
As told in above answer new DB is not required, but we suggest clients to do because many clients using database for long time and upgrade is good time for them to update/change.
Q3. If you are setting up a new DB and some how moving the existing Db assets to new DB , what is the use of running the DB migration on a DB which is already in the latest version?
This is answered in Q1 & Q2
Q4. As per my knowledge, uses just need to use the target version Database component Configurator and migrate the existing DB to the target DB. The scripts are designed as specific to each path and user hardly needs to comment any lines of scripts (Apart from a special case of TN, where user needs to comment few lines if some specific DB fix is already applied to the source 7.1.3 DB)
I don’t understand question here migrating existing DB to Target DB. DCC is not desgined to migrate existing DB to Target DB, Running migration scripts mean existing DB will get inserted/altered with new tables as required by 9.0