Upgrading from "pirated" vB to purchased

[Angels1928]

Before this gets flagged, deleted, or whatever, please just read it through

To start, I'll give you the low-down:

I moderate a large vBulletin forum, along with one (1) member of limited Administrator status. The website gets a lot of traffic, and is very popular--but the owner has disappeared for over 8 months, and the hosting is about to die.

The other admin and I aren't going to let that happen, and we're transferring the entire website to mine. As neither of us have access to the "server side", the other Admin only managed to get a hold of the MySQL database (over 200mb), and with that, I've transferred the entire "old" website, to the "new" one--addons and all, fully working.

The vBulletin that had to be backed up was 3.5.3--a legit, purchased version. To restore it to my website, and make sure everything works, I had to use a nulled version of 3.5.3. We of course aren't going to keep using a nulled version on this website, so we're getting donations to purchase a copy from vB.com.

My question is, how do I upgrade from the nulled version to the legit version, once I purchase it? Will it involve just overriding files, or perhaps I run a .php file (in the legit version) which has me put in my license details?

Thanks guys,
Angels