Thanks Robin, this is exactly what I intended to do. Unfortunately that was not that easy.
First i decided to import via PHPMyAdmin the entire “user” and “user_meta” tables (just in case some users unsubscribed or changed their password meannwhile)
Then i entirely deleted (delete from trash as well) from my copy-prod website the oldest replies i already had on the beta website. In order to export/import to beta (the fact that you have to reassign one by one each author could be a big hassle if your forum has been very active)
Then i entirely deleted (delete from trash as well) from my copy-prod website the oldest subjects i already have on the beta website (note: dont remove subjects before replies). In order to export/import to beta.
I’m fortunate i did not have to deal with new forums.
I still have some inconsistencies to deal with, but overall it seems to work.
Next time i’ll need a beta website with bbpress I’ll definitively find another way.