This is one of those do what you want situations.
When it comes to my own rings, I'm more in line with your coworkers point of view. I feel like the ring he proposed with will always be the genuine engagement ring and the one I start wearing on my wedding day will be the one and only wedding band. Stand-ins and replacements wouldn't be the same to me because they wouldn't be the ones that were present in those moments. However, I understand life happens. Things get lost and damaged and if that happened I'd replace them and refer to the new ones as my engagement/wedding rings. I can't imagine I would ever willingly choose to upgrade or replace my rings, though. Granted, the newness of my ering hasn't worn off yet, and I haven't even started wearing the wedding ring, but still. I'm the type of person to get attached to things with sentimental value. Everyday I still wear two inexpensive rings that he gave me 15+ years ago because they mean something to me, so I think I'm pretty safe in predicting that I wouldn't mess with my wedding set. The diamonds in my ring are plenty big for my tastes and the style is a simple three-stone trellis setting, so I don't see it ever looking particularly dated in the future. If I ever do find myself wanting something flashier or liking a different style, I'll just buy something to wear as a right hand ring.