Is it possible to upgrade from NewBB 1.6 to CBB(newbb) 3.08?
Will the data / files/ forum posts be lost?

Pls help ....

Yes, perfectly possible. How depends upon your XOOPS version.

xoops 2.0.x:
1. Download xoopseditors 1.21 and Frameworks 1.22 from xoopsforge.com and copy them to your website here:

xoops_root/Frameworks
xoops_root/class/xoopseditor

2. Delete the current xoops_root/modules/newbb folder and replace it with the /newbb folder from the CBB 3.08 download.

Login, go to Modules Admin and update the module.

That's it. Job done

xoops 2.3.2a

Follow step 2. above. xoopseditors and Frameworks already in place (although you could upload the "Frameworks for XOOPS 2.3+" to be sure - search on here).

WARNING:
DO NOT DO ANY OF THE ABOVE WITH XOOPS 2.3.1 as there was a problem with synching/updating the Forums which caused posts to be lost/orphaned.

This has been cured in the 2.3.2a release but, as ever, it is always wise to backup your database before carrying out upgrades.

HTH