Hi! So I am not an expert, and don't work at a University or in admissions. But I did take five years between undergrad and my MPA, and it has been 10 years since my MPA for my applications this year for a PhD. First, if you are even thinking about it, reach out to professors you liked. Yes, from 15 years ago. Did you have a favorite professor? Reach out and catch up, have a zoom call. And do this a full year before you are going to apply. I think you can reach out under the frame of "I'm interested in applying for X and I loved Y that we did *remind them who you are, and I'd love to speak with you about Z". If you are changing fields, I would try to get some experience in it. Even just volunteering, or a part-time job, with a supervisor who could be another letter. Last time I have a mix of previous professors and my boss. This time I have previous professors from my masters, and through them I connected with a professor in the field I'm moving into, and developed a relationship over a year+ focusing on my research goals etc. This last letter can speak to how I will do in the new field, the first two can speak to how I think, work and learn.
hope this helps! Congrats on wanting to go back to school, it is a big project and particularly hard when you are bit older and out of the system.