Yes, U-Boot environment variables control these, both while U-Boot is running and after linux boots. There are plenty of basic references available. I recommend Embedded Linux Primer by Hallinan; your current questions would come under sec 5.3 and chap 7.
At U-boot prompt, do "printenv", you will find U-Boot has set these variables for itself: stderr=serial stdin=serial stdout=serial
As long as you do not do "saveenv", you can change stdin and stdout of U-Boot. Probably you will find those changes quite nonuseful. Reboot to get the previous settings back.
You can change baudrate very easily setenv baudrate 9600
Initial Linux console is set by an item within "bootargs" variable, e.g. ...console=ttyAMA0,115200n8... If you want different baudrate, try something like this at U-Boot coli: setenv bootargs console=ttyAMA0,9600n8 ... (copy what was previously there for other items) saveenv
After linux has come up, various other console options are possible but are somewhat advanced, good luck.