Hi! I'll share my thoughts...
I upgraded. No regrets. I love my ring!
My story to share...
My husband is like yours, and he loved the idea of me upgrading my ring. Like your husband, he felt it was a reflection of our "growth", using your words. When I mentioned to my husband I wanted to upgrade, my husband actually said "yes, you need a bigger ring!"

Unlike your husband though, my husband was sentimental about my first engagement ring, so he wanted to keep it, which I did.
I am like you on Tiffany... I love Tiffany! I purchased my new Tiffany ring and love it as does my husband.
For my upgrade, I still wanted it in a simple setting as I wanted the stone to be the star. I wanted a big stone, but not a big ring with side stones, thick shank, etc. To be sure, I also look at other other brands and loose non-branded diamonds (took me over a year).
In the end, the Tiffany diamond sparkled like mad, and I love the simple beauty of the Tiffany setting. As you stated, the Tiffany setting is often copied, but it is never duplicated. Yes, Tiffany is ridiculously overpriced, but it's worth it to me. The diamond sparkles like crazy. By the way, I wear the super thin 2mm knife-edge wedding band (you can tell I want my left hand to be all about my diamond). I actually wanted to skip wearing a wedding band, but I would look like I'm forever engaged.

What happened to my original e-ring?
It sat in my jewelry box for the longest time. I wanted to sell it, but my husband said it's sunk cost. Okay, peace.
One day, I was getting a custom piece made, and my husband had this light-bulb idea and said "hey, let's use the diamond from your first engagement ring as a side stone!" Done! My husband is also super happy he came up with the idea.

I know you want to sell your engagement ring. I think it's perfectly fine especially since your husband is okay with it. He was the one who bought it for you, and if he is okay with it, that's what matters.
I think it's wonderful to get an upgrade. I am not a believer one should wear the same jewelry that you wore in your youth. We all grow, and in the context of jewelry, I like my jewelry to reflect my growth and who I am now.
Both of you can stare are your new ring, have huge ear-to-ear grins, and say "We've grown together and come far."
I say go for it! You will love your new ring and your husband will too!
By the way, Tiffany will give you the price you paid for your ring towards an upgrade of a ring twice the price (e.g. if you paid $50K for your original ring, you will need to buy a ring $100K or greater). You may want to check though if the rules have changed.
Hope this helps in your decision. Good luck!