If you would like an upgrade via a lab grown diamond, I don’t see a problem with that
whether and when to upgrade is solely your choice; and I don’t have a problem with that either
I don’t think there should be rules on what makes you happy
full disclosure: my DH and I have been married for over 30 years. In the frist few months, he asked if I minded if he didn’t wear his ring (he found a ring uncomfortable). In the subsequent years, I’ve lost a variety of engagement rings (including a sapphire that we believe DH inadvertently threw out with newspapers), so I’ve used a small assortment of family rings to stand in for an engagement one. So, IMO, you should just shop around and buy what you love and what makes you happy

The last time we were in Bulgari, DH thought a small serpenti ring would be more me than the usual wedding and engagement, so recently, that’s what I’ve been wearing.
also, I have to say that I find the De Beers three months of salary thing kind of offensive. People have essential expenses when they marry like real estate, cars, childrens expenses, retirement. A ring can be anything. If someone had a sentimental attachment to a Cracker Jack ring, more power to them.
I do have sentimental attachment to some jewelry DH has given me, but an engagement ring isn’t one of them. JMO of course