First of all, i'd like to ask, if there can be any problems updating a store from v1.6CE to 22.214.171.124CE?
I know I have to check if all the modules using currently are exists on the newer version. What about the theme?
And what's the best solution for upgrade if I want to use a dummy-site for the upgrade procedure? (move the original, upgrade, move back to the default domain)
In my experience I have upgraded Magento 1.3 version to Magento 126.96.36.199 version. I think if you follow proper steps and do extensive testing it should not be much difficult. Though you can face issue based on number of custom modules used.Deprecated functionalities etc.
You must do the upgrade on the development environment separate from your production site.