We created a TM with British Englis as a source language, but it turns out that the client now wants it to be US English. Since we have already added a lot of segments I was wondering if it is possible to simply change the name of the source language.
Thanks a lot
The best solution would be to create a new TM in the languages you want, then export your en(GB) to a TMX. Now import the TMX into the en(US) you created.
There are other solutions but if the variant is important for you then that is a safer way because the TM also contains various language resources which may or may not contain things specific to the flavour your originally selected. In practice there may be little to no difference but this method would avoid any doubt.