8.2.1 to 9 Upgrade is a nightmare

[NemesisNG]

Been using Prestashop for a number of year now and upgrading is always a difficult task. Using a purchased theme which never retains any settings is the main problem, but I always just have to spend days setting it all up again.

I've tried a great number of times to upgrade from 8.2.1 to 9 and it never fully works. First main issues were during the upgrade process itself it was failing half way through and ending in an error 500, but always related to a module from the theme I'm using. So I updated the theme to an apparent version 9 compatible version, but that made no difference.

I then removed the theme totally and every module associated with it, the upgrade then for the first time completed, but now in the admin if I click on the Module tab and Module Manager I just get a blank white page, so there's still something wrong.

Why is it so difficult to ever upgrade this software without some major issues?

[JBW]

You need to deactivate all (non-native) modules by renaming their folders on the server as even deactivates modules will be initiated when opening e..g module manager. By renaming them one-by-one you will find the incompatible ones. Before any upgrade you have the ensure that all modules and the theme are updated to new compatible version. 

44 minutes ago, NemesisNG said:

Why is it so difficult to ever upgrade this software without some major issues?

In my experience 95% of the upgrade failures are caused by external modules or wrong server settings (e.g. PHP versions)

[ComGrafPL]

1 hour ago, NemesisNG said:

Been using Prestashop for a number of year now and upgrading is always a difficult task. Using a purchased theme which never retains any settings is the main problem, but I always just have to spend days setting it all up again.

I've tried a great number of times to upgrade from 8.2.1 to 9 and it never fully works. First main issues were during the upgrade process itself it was failing half way through and ending in an error 500, but always related to a module from the theme I'm using. So I updated the theme to an apparent version 9 compatible version, but that made no difference.

I then removed the theme totally and every module associated with it, the upgrade then for the first time completed, but now in the admin if I click on the Module tab and Module Manager I just get a blank white page, so there's still something wrong.

Why is it so difficult to ever upgrade this software without some major issues?

Easier is same as it was with 1.7 - 8,  create clean installation and do the data migration, with updated theme, modules.