The content structure is quite complex, it's easier to just switch culture on your language rather than changing language on your content.
First, go to admin and enable en-GB.
Then go to tblLanguageBranches, look at the rows that has the names "en" and "en-GB". For example "en" have value 1 and "en-GB" has value 10.
All content refer to the language "en" by referring to the Id 1.
Change values or these rows except for that the Id so in the end "en-GB" will have the Id 1, therefore all content that previously was "en" will now be "en-GB".
Read more at Fredrik Haglund's blog http://blog.fredrikhaglund.se/blog/2007/12/04/episerver-globalization-and-dynamic-properties/
Hacking the database like Alf says is unfortunately the way to go. This is the only time you will hear me say this combined with Episerver.
We have created a site with the language set to en, however we'd like it to be en-gb so we can facilitate further translations going forward.
Quick question: is there a "painless" way to translate all existing content (commerce too) to be the new required language? I've had a dive into [tblContentLanguage] but it's proving to be futile.