Moving nopCommerce Question

Hi all long question I'm afraid,
I was a little naive last time I upgraded from nop1.9 to nop3.3.
See I had a heavily customized version of nop1.9 with a seriously customized DB as loads of other apps for external inventory, tracking, admin even human resources hung off the DB and it had grown.

Well when we upgraded the last time 95% of customers for us are guests users I really ignored the other 5% through lack of knowledge and just did a new install and manually ETL'd the data I needed to the new setup.

Everything was great except the users who had stored passwords on the DB and in cookie's in browsers, they couldn't connect and needed to drop cookies to access again.

It was a bleedin' nightmare, you'd think we'd learn wouldn't you?

We haven't and now again have a heavily customized nop3.3 and DB with even more apps hanging off it. We are in the process of discussing another upgrade and wanted a little guidance this time.

We have discounted doing the nop DB scripts as we really do customize the DB and we will run into endless issues. The only way we can see is to do a fresh install and again ETL the data we need which generally works fine but again the password and existing logins I'm worried about. How can I move this data too so the logins are seamless?

Any help / ideas would be gratefully received.

Thanks
Rob

Can't you first run the nop DB scripts, and then run your own custom scripts?

I have as I said really customized the DB and I have my doubts the scripts will work well.
I want to move to a fresh install of 3.7 and manually load up the customers I have on my DB, surely this is possible?

Doing this I can then create the functionality I need on a separate DB so I don't populate this new install.