A student I mentored years ago came back wanting letters of recommendation for graduate school. Student had a low GPA for entering grad school due to some health issues, but they were real and have since fed this student's work and research interests. In short, I could easily see how all the parts of this person's life were tying together and made a Master's degree make absolute sense and would increase his/her earning power. Cool. I said so in the letter.
Student got into several very good graduate programs despite the somewhat low GPA. Cool.
Now student comes back at the very last minute wanting another letter to take his/her life in an entirely different direction that makes absolutely no sense at all. I'm swamped with moving and a major change in my career. They have no way of knowing this, but still, it's there. I don't want to just say no to the letter (which would have to be recrafted), I want to drive to where this student lives and shake them till their eyes bleed. I mean, for FUCK'S SAKE. You got into better graduate schools than I did with a shitty ass GPA. You had an honest to god AWESOME plan for research and everybody who read it SAW THAT.
No, I have no idea how to write a letter for CompletelyDifferentField and I have no idea why you'd want to do that. I don't know the employment statistics. They might well be better than my current field at large, but what you want to do is still in high demand. I think you'd be fabulous at it. WTF is going on and why do you think that the end of July is the right time to make it go on?