Create a dev site so even if it breaks that won't be an issue.

Deactivate all extensions apart from K2 and the ones that support J! 2.5 or 3.1.
Update all these extensions to their latest version.
Use the jUpgrade method or Admin Tools to upgrade your installation.

Now you will have a Joomla! 2.5 site, proceed again with the same procedure to upgrade to 3.1 if you wish.

At this point since you only have a light/bare Joomla!/K2 installation and it should be completely functional in the backend. (I do not know about the template).\

All you need to do is some trial and error... Activate all other extensions one by one and the ones that are crashing your site need to be reinstalled.

Again... do this on your dev site.

Great !!!! It worked. Not perfectly. But most stuff are there. The language field gave a problem that I solved manually(*).

For anyone interested, the exact procedure I used is :

1) Take a copy of my prod site

2) Update all the extensions I could, deactivate the tiny ones without J!2.5 equivalent or that should be migrated or that I could easily re-install and reconfigure on the J!2.5 site

3) With AdminTool, run all their cleaning actions

4) Check in my DB (with phpMyAdmin) if I had no unused tables (eg tables from old extensions) and drop them

5) install and run jUpgrade (and not jUpgradePro)

6) I had to remove some other extensions because jUpgrade got stuck

At this stage the we have a J!2.5 site. The next actions will be in this new site.

6) Install extensions (and K2 extension)
7) With phpMyAdmin check every K2 tables if their items have a "language" field. If yes and if empty, update them with "*".

8.) Check all menus pointing to k2 items. Actually, I dropped them and recreate them.

Voilà. That's almost it.

Next steps are templates (and K2 templates) migration.





What about setting a default value for that field in the MySQL tables in the next K2 release ?

Actually there was no need to remove K2, you could have upgraded using jUpgrade with K2 enabled.

Other than that, well done.

The asterisk is there in the newest versions, that is why I said you didn't need to unistall K2.

I didn't uninstall nor deactivate K2 !
jUpgrade copied all K2 data (db + images), but the component itself was not visible in the Joomla's component menu nor in the installed extensions. That's why I had to reinstall it. This kept (thankfully) all migrated data !