UPSC gives you complete liberty to choose any optional subject you feel comfortable with. In case, you are planning to take Geography as optional subject in Civil Services (Main) Examination, there is no reason to have any kind of apprehensions.
Geography is one of the most popular optional subject and semi-scientific in nature. Lots of candidates make cross-domain shift while picking optional subject and give outstanding performance; so, you are no exception who is thinking about it.
Go ahead and prepare diligently so that you are able to do justice with your choice.