My folks are in their 70s. I told them a few days before I was going to transition. I was scared poopless but I sat down with them and told them I had been depressed, that I had gender dysphoria, that I had been seeing a psyche, that I was transgender, I had been on hormone therapy for almost a year and that I was going to transition to female the next week. I showed them some pictures of myself as Grace so they had some idea what I was talking about. They were pretty stunned, they said that they were accepting but that changed within a few days as the reality of it sunk in. I was able to swing my mother back around by having lunch with her shortly afterwards and that helped quell many of her concerns. My father continues to struggle.
Since you have ten days with them maybe tell them on the first or second day and then see if they will meet you as Sarah if you are ready for that to be a thing. That way you'll have a week or so to try and sort out any misunderstandings or problems.