Tough call. I am going in the other direction and can understand it has not been easy for my wife. Then again, if she transitioned? What would I feel?
I suggest that everyone’s feelings and responses are individual and there is no ‘correct’ response. Of course you and your partner have a long history and she/he will still be the same person inside. How important are the looks? For me, the bottom line that I would find difficult to deal with is if my wife decided on a beard. THAT would certainly raise questions, but in fairness neither would I find it easy to throw away three decades of relationship either.
When I broached my transition, initially my wife wanted a divorce, but said she would wait until I had completed my transition and was safe and OK. As it happens, that seems no longer an issue and she has got used to Aija. I am sure it is not an easy situation for either of you and certainly if its early days anything could happen. The transition could be partial or not happen at all or could be complete.
The fact you are seeking advice shows to me that you, unsurprisingly are shocked/surprised, but care. That is a healthy sign and maybe you can work through it together.