Hi
pwecious_323, prior to my upgrade, I had a 0.91 carat round brilliant D VVS1. So for my upgrade, my priority was that it must be bigger than 1 carat so I was looking at stone sizes around 2 to 2.25 carats. Also, I wanted a fancy cut diamond for my upgrade and I really like the look of a cushion cut diamond set in diamond micropave halo.
I still care about color and clarity so to upgrade in size, I chose to go down in color but still in the colorless range i.e. between D to F color and clarity but not anything beyond VS2 clarity. Regardless of the 4Cs or certification may be one thing but at the end of the day, you have to see the stone and like what you see. Like many have said on the forum, you aren't going to carry the certification around yeah?
So after trying on different stone sizes for comparisons, I ended up with a 1.82 carats F VS2. The stone was just right and within our price range. Coincidentally, it's 2x the size of my original e-ring so that in itself feels special !
I got my upgrade about 6+ years after marriage, at no specific special occasion. There is no general rule of thumb for an upgrade, most important, you and DH are ready to take the plunge and have the budget for it, then go for it!!!!!
Good luck and have fun!