Working weekends is a must at my establishment. Non negotiable. I say that in interviews.
Short answer is "No". You are not the only one with this kind of problem. It is tough when you have a "vet" who needs to change their schedule. However, if the person is trustworthy, and I assume they are since they have been with you for 3 years, I would try to work with them to prevent losing them. I find it easier to find a parttime person to work weekends. Students are definitely interested in this type of schedule.
Perhaps he/she can continue to work the schedule given until a new person is trained to take over. I would try to work out something that is best for all and makes everyone happy. Last thing you want is a disgruntled employee on staff. One bad apple...
Funny--I'm giving a talk at Coffee Fest NYC about just this issue! I think employee retention is frequently overlooked in the industry and operators accept high rates of turnover without addressing the issues that cause it. I think keeping baristas engaged and motivated is a huge issue in cafes. You mention the schedule; is that your biggest reason for losing employees? It's certainly not unusual to require each employee to work one weekend shift and one closing shift per week. Is the employee in question a student, does she have another job, what's causing the sudden schedule issue?
Yeah--no Coffee Fest in Seattle or Portland this year! If you want to wait for a west coast show, it'll be in Anaheim in September.