mBtc is what you mean? Changing the value of 1 BTC as you told it is more than change of name, but more like a stock split and very hard to do.
Its a simple math thing, to multipy by 1000, in order to move the decimal place 3 places to the right.
I don't know why this would be difficult, but I don't know much about the code for bitcoin.
It does depend on whether this can be done in the code. It seems like a simple thing to me.
Mtgox is thinking about it, as can be seen at their graph site at bitcoinity.org/markets
where they are using the mBTC now.
The client already supports mBTC. It is in the options. MtGox does not own or operate bitcoinity. Bitcoinity did that on their own.