Upgrade Question from vB3.8 to vB4.2!

[ChiNa]

Hi everyone, I have a few questions about upgrading to vB4.x! Now I did try to upgrade and it was not that easy, but here is my problem! My forum is using to many plugins and addons and after upgrading for testing, I realised that it may be a bad idea, and here is my reason why! I have lots of Attachments and thumbnails and later this year I changed my Attachments from database to use a custom folder,,

I did try to upgrade, the full upgrade was a success, but now I had a lot of stuff not fitting together, but I did success at the end by uninstalling all my mods and plugins! And then I tried another trick, I simpy tried to only import my threads, Posts , users and attachments, which i didnt success..

I have now 2 simple 2 Questions to ask, If I should do it all over again:

First Question: I wanna ask If I can change back all my Thumbnails and other Attachments to be put back into my DATABASE like it was befor! And THEN upgrade without having any problem?

Second Question: The other question is, Can I import my USERS, POST, THREADS and ATTACHMENTS only, instead of doing a full upgrade! I know that I have to disable all my plugins or uninstalled them, but now i decided only to Import my USERS, THREADS, POSTS and ATTACHMENTS ofcourse!... Is it possible to do that? If yes then whats the easiest way, or strategy?

Do I have to do a full upgrade first to be able to do that, or can I simply do it one by one?

Thank you very much in advance.