Found old retainer... safe to wear?

Braces taken off 9 years ago. There are obvious gaps that have formed on my upper teeth. ( 1- 2mm ) I only have my upper retainer and it actually fits... Tight but fits. I called and told my original ortho office, and spoke to the receptionist explaining I haven't worn my retainer in 7 years and she said just wear my retainer anyways and it should shift back. I have it on now and it does hurt. But is this wise to do especially as I'm only wearing one retainer and will it shift back?