Mmmm I still don't understand if it will be a good move to purchase this board or not.
At least I will do like the other member suggests, so I will purchase the new board, but after that I will wait for the develop before proceed with the installation. I am too much afraid to loose important parts of the site.

It will be possible, once bought the new board to run some testm just for the staff, while the old board is still running?

Ones of the worst thing is that last week after years of waiting I installed a new theme for my board in order to replace the default theme. From this post it seems clear that the 3.8 theme will not be ok for the 4.0 board.

What a pity, what a mess.

Anyway there will be some special efforts to convert the major modification from 3.8 to 4.0?

And the last question is: what do you suggest to do, to a user of vBullettin with a quite big site (85000 members).

Special efforts by whom? It is up to the developer to decide whether to upgrade their modification. Someone else cannot come along and update the modification and then posted here *unless* they get permission from the original developer. You may, however, convert mods on your site for your own personal use (but not distribute them).
I suggest you set up a test site, take a backup of your live database and upgrade it on the test site and then fix/upgrade any modifications you may need. That is what I am doing anyway.

Thanks for the answer lynne.

Here you are another question:
What will be the advantages to pass from 3.X to 4.X?

thanks

That's a hard question to answer. It really depends. If you like to stay up-to-date with the latest code, then you'll want to upgrade. And, I know there are lots of plans to add new features once 4.0 is out. Some devs may move up to 4.0 and stop their 3.8 mods. Or vice versa. I think everyone needs to evaluate their own situation in order to decide what is best for their site(s).

Upgrade done, now I can just cross my fingers :D