Glacier White Corian wouldn't at all be a waste and would save you money over quartz right now with the tariffs on Chinese imports if you purchased a non-American brand which is basically anything except Cambria, and Cambria itself is expensive. You can even get the Glacier White Corian in a 3/4" thickness (most other colors only come in the 1/2" thickness). You might want to read up on some of the durability issues between the two surfaces. For starters, quartz can change color with exposure to sunlight which is something you don't see with Corian, and quartz can melt at lower temperatures (150 degrees Fahrenheit) than Corian (380 to 400 degrees Fahrenheit). Corian can be resurfaced easily, while this is much harder with quartz. Corian will develop a patina of light scuffs of which you are already aware, but again these can be buffed out whereas scratches in quartz aren't as easy to make, but if they are made, they're hard to repair inconspicuously.

If your base cabinets are dramatically unlevel, then you could consider placing a narrow wood trim piece underneath your new countertop to disguise the shims. You'll want the surface level within 1/8" for any countertop installation including Corian. While Corian isn't as heavy as quartz or natural stone, it can still crack when placed on an uneven base. Another option would be to have the edge mitered to hide shimming. In any case, leveling can be done from the top as the previous commenter pictured.

I replaced laminate countertops with granite in my last house. We had old custom cabinets and a tile floor. There was a lot of settling in the house and the floors showed it. The kitchen was over a crawlspace so my husband jacked up the floors which improved things. From there the countertop guys used shims. There was a gap in one spot between the counter and cabinets and I used a piece of 1/4round to trim it out. My cabinets were painted so I painted the trim to match. It looked fine, but I'm not sure I would do it again. I think I got lucky that the gap was in only one spot and the trim board fit all around the counter without blocking a drawer. This would not have worked with full overlay cabinets. I never realized how great laminate countertops were at hiding imperfections until I installed the granite. My recommendation is to look at the nice laminate.

My Corian is already shimmed. You can't see it because of the edge.

Dormelles, that is a terrific point you about the cost and the tariffs.

Thanks so much, you've given me a lot to look into and think about.

Have you looked at the current corian colors? While glacier white is a classic choice, you might be pleasantly surprised with their range of offerings.

You'll need a counter installer to give you an evaluation of your install. Get some quotes. You could start with pictures highlighting any areas out of level.

A good installer can level the countertop with shims. I love my quartz - made in the US. There are many possible countertop choices out there. Enjoy your remodel:)

I find your post very interesting because I have the same issue ( small kitchen with uppers that don’t go to the ceiling). The cabinets are in decent shape and it never occurred to me to replace just the uppers. How did it turn out? Wondering if you are pleased with the result.

Pamelamcaleod, I didn't do it yet. Sadly my dad who had been fighting cancer for two years got very sick and was put into hospice. I moved in with my parents for awhile to help take care of him until he passed away. I am just now picking this back up. I'll let you know what happens. It might be sometime. We actually broke a bathtub so we are dealing with replacing that bad boy at the moment lol

So sorry about the loss of your dad. Our lives mirror each other. Good luck dealing with your Reno’s.