First time upgrade from 1.5.6

I'm trying to update my website from 1.5.6 to the newly released 2.0.3.1.
I have some doubts-
1. Over the course of last 1 year, I've made changes to files in admin, catalog folders and css files, but mostly to the 'language' folder. What will happen to those changes once I upgrade? This is my first time upgrading opencart. I did use vqmod (v 2.4.1) for some changes, but assume that the number of those changes are negligible compared to direct changes in files. If the files will be lost, I might need to spend some weeks re-doing those changes to this version, and probably spending more time with vqmod this time so it doesn't happen during future upgrades (I still don't get vqmod, nor am I particularly good at it, so I'd like to avoid it if possible).
I'm guessing the products, images and customer database won't be lost on upgrade, so at least that's safe.

2. From what i've read so far, there are 2 ways to upgrade-
a. http://forum.opencart.com/viewtopic.php?f=135&t=139838
b. http://www.opencart.com/index.php?route ... n_id=19572
Which of them is the easier option to choose? Also which is the safer one, the one that hopefully won't remove the changes I've made over the past year'?

Any help would be highly appreciated.