I have inherited a Omeka-S 1.4.0 project and I wish to upgrade it to 2.1.2 version. I have search in the Forum and seen the differences in the DB structures but Iḿ not pretty sure which is the ideal way to migrate all of the contents without any loss from 1.4.0 to 2.1.2, please could you give suggestions?
You should just be able to follow the Updating instructions on the Omeka S manual. Doing the Omeka S upgrade process and then just updating the modules you’re using should be all you need to do.
It’s a good idea to make sure you have a backup before starting the upgrade process so you can always just return to your 1.4.0 install if you have issues.
Earlier this summer we did the migration you are contemplating and as @jflatnes says, it kinda Just Worked. We only had mods in Themes, if that matters.