Hello Johann,
As a tip, I would export and import the data sets without "default provider", so you only export and import all your own foreground processes (they will remain interlinked between each other) but without the set default provider from the ecoinvent database. This way you will not create duplicates and you will not mix up different ecoinvent versions.
On the other hand, if you want to make the process names distinguishable, before you import data into ecoinvent 3.9, you could also write in the SQL editor something like:
UPDATE TBL_PROCESSES SET NAME = NAME || ' ecoinvent 3.9';
to add to all process names in ecoinvent 3.9 the string "ecoinvent 3.9".