Why did you upgrade?
I wasn't happy with the EM1 IQ for large prints, or even when using them as 1080p desktop backgrounds. The images never seemed "crisp" to me, and I wanted more Megapixels. Crop sensors tend to top out around 24MP so I looked at full frame options. After a VERY long consideration (3-4 months) I picked the D810 because it was tough, high MP count, and better value than some other options.
What did you upgrade from and to?
Olympus EM1 to Nikon D810
Did you immediately notice a significant improvement in the quality of your photos or was it basically the same?
I'd rephrase that to noticing a significant improvement in
image quality. Obviously a better camera isn't going to improve your photos, but yes, actually I was surprised by the improvement in IQ. That "crispness" showed straight away, with much more detail and sharpness, and much cleaner images as well.
Was it worth the investment?
It was a LOT of money, but so far I think it has been worth it. I was originally considering adding a Fuji system to my kit for travel, but to be honest the D810 with a couple of primes is more than manageable so I probably won't bother now.
Edited to fit the question structure