I think the upgrade policy is worthwhile if you’re the kind of person who will chop and change / go bigger and bigger (in an MRB) and is not particularly sentimental about the original stone. For example, the pattern in my social circles that I’ve seen is to stick to the ring you were proposed to with, and if you do want bigger, at a much later significant anniversary or birthday to perhaps buy a much bigger stone and wear both. Or to have multiple engagement rings. But I don’t know a single person who traded their e-ring back. At best they wore it as a pendant, or gave it to their son to propose with. So I can’t see the upgrade policy making a lot of sense here, since it’s something that’s “not done”.
I like the idea that you’re able to easily recoup what you paid, since an upgrade policy is the best way to get trade in value. However, the cost of the upgrade policy is built into the price of every stone, so if you dont utilise it that’s money you’ve paid unnecessarily. Super ideals are at a premium not just because of the extra wasted rough it takes to cut for precision, but also because the sellers have excellent upgrade and return policies. I think “grass is greener” syndrome exists amongst people in varying amounts anyway, so having a way to mitigate it isn’t bad as long as it doesn’t impact someone mentally in a bad way.
So the biggest pro is the ability to swap out your MRB as much as you like (given that they have adequate inventory, at a certain size that won’t be available/will be unaffordable at the premium)
The biggest con is price related - having to get a new setting / head for your setting (dependent on the diameter difference between old/new and the tolerances of your setting). Also that you’re only able to go for MRBs pretty much.