people's view on replacing engagement rings?

so if you get engaged at a time on you're life where you can only afford a $50 or whatever ring..... then fast foreward a few years and you can afford to get a new one.... ready for the wedding maybe? a nicer maybe more expensive one?? or maybe the old one was a little worn or tarnished???

what are people's views on this?? do you think it would be ok? or do you think women should just be happy with the first ring they get?