In theory. It doesn't happen often past initial launch of a product.

If you can build a plausible case that you did this (eg. simply making your fw image smaller justifies using a smaller eMMC chip), and provide a few benchmarks that demonstrate equivalent performance in those scenarios, you'd be of the hook in any legal mandate to keep the performance the same even if your new hw revision ships with weaker hardware.

This is even a common product development strategy: ship to market asap, optimize the margins later.