Recently added album "new" flag identification, how to reset it?

I converted a bunch of my AIFF to ALAC file to make more space, and Roon detect all those album as new newly added in database (flag them with a “new” tag image in the left/top corner of album image) … there’s a way to reset this vignette without playing all of them one by one, or by re-import all my lib again and needed to re-identify more than 300 albums manually (that is out of question) ?

No. That flag is automatic and will go away after a short time. You can’t turn it on or off, or otherwise influence it.