It really boils down to how much the two of you value your current relationship and value its future. Do you value this new tran experience higher than what you have now with your girlfriend? More than you value your future with her? Is it worth the possibility of loosing this wonderful relationship you have now? Ask yourself, because you may loose something very valuable.
If you do think that having this tran experience has a higher value than your girlfriend experience also ask if she values your relationship. You are not the only one loosing should you break up.
Think, or ask others, about ways to broach the topic only after you are pleased with the answers to the above.