Hello,
I summary procedure when we make an upgrading from 1,8 (or 1.7) to 1.9. I propose fresh installation 1.9 because it won't effect to live website. We just configure fresh new with all configuration, theme, extension,... from 1.8 (or 1.7). Then, check if working properly, then switch to new one. We can roll back to 1.8 (or 1.7) if 1.9 doesn't work properly. I summary the procedure:
I. Preparing
1. Export database 1.8
2. If you have theme, extension, please note so that we wont forget them
3. Download magento 1.9, extract and upload to new folder.
II. Installtion
1. Install magento 1.9 with database
2. install extension, theme and configure website like 1.8
3. Testing III. Switching to new one : Reconfigure the website path, point to new folder 1.9
Completed
Regards,
Vu Le