I want to move the exact setting(all what its in the baord setting and users) from RB450 to RB450G board. What should I do?

You can use Export command, make necessary changes and then Import it to the other one

more precisely, I recommend "/export compact file=name" command. "Compact" is important, as it only exports the changes that you made.

You can use Export command, make necessary changes and then Import it to the other one

what are the necessary changes. Do I need to only change the mac addresses?

Actually you don't need to do any changes if you run this command instead:
this command will only export the configuration that you applied yourself. this will work when importing to other routers.

more precisely, I recommend "/export compact file=name" command. "Compact" is important, as it only exports the changes that you made.

Is the compact file is enough. because I notice that I might not need to change anything but the file size is smaller that the one you get from "/export file=name"

open the file in a text editor and you will see the difference.

export file=name saves ALL configuration, including RouterOS default values
export compact file=name only saves user applied configuration. it's enough to migrate all config to another device, since the other things are defaults anyway

more precisely, I recommend "/export compact file=name" command. "Compact" is important, as it only exports the changes that you made.

and in the other board should I do

/import file=name

or there is /import compact file=name

I believe will do.