Update: This tutorial is not useful for Thesis 2.0

When I purchased license of Thesis theme version 1.7 was already out. So I had not to upgrade for awhile but after latest version of thesis 1.8 I had to upgrade for new version sooner or later. But for me there was a problem how to upgrade to new version without losing my all customizations and settings.

I started searching and hopefully I found nice tutorials, all the tutorials works for all new versions.

Preparation:
If you think you are ready to upgrade for new version of thesis theme.
Download the latest version from DIYThemes.com, there you will find instructions for upgrading.
Thesis provides you a very easier way to upgrade to new version so you do not have to worry. If you have downloaded the theme, it will be in zip so extract it. There will be a folder of same name inside a folder as a folder thesis_18 will be inside thesis_18 folder. You have to copy or paste that theme or folder on desktop.
In that folder you will find a folder “custom-sample”, rename it to “custom”.
Now it is time to upload your theme to your blog. You can use FTP or you can direct upload using admin panel.
Now you have to download your custom folder from old version (all the folder settings are same for new and old versions) of thesis theme that you can download using FTP. After downloading your old thesis’s custom folder upload it to new version of thesis theme folder. FTP client will ask you to overwrite files because there is already a folder with the same name, you have to click on yes. When you are done you can activate your new version of thesis theme.