What will happen if I will insert a new and blank atmega micro controller on the UNO board instead of
the original one that came with the board from factory ?
It will sit there looking pretty.
will I be able to download the sketch into it and the board will work as usual
with the new sketch ?
No. First, you will need to install a bootloader on the new chip. You need either a 2nd Arduino or an external programmer to do this.