I will need to do a similar thing eventually on my IMX6UL-DART, so perhaps we can share some ideas.
I remember successfully moving the Linux debug console from ttymxc0 to ttymxc2. I had to change one of the .dts or .dtsi files and re-generate the corresponding .dtb file, then change the linux command line to contain 'console=ttymxc2,115200 ...'
1. Do you want to disable the uart debug console in U-Boot, Linux, or both?
2. Are you getting the compilation errors when you modify the .dts file?