Alter table --- big table

Hello all,

I'm having an issue here at work.
So the problem is we have a set fo 4 tables that are very big.
Lets take the biggest 125gb.

I have to alter table column from a smalldatetime to a datetime.
My first question is:
Since this is an implicit conversion - no data should be copied, right ?
-- after checking query plan, 1º copy of data then 2º update catalog.

Second question:
I don't have nearly enough space left to preform the previous, so my option is:
1. drop indexes and triggers
2. bcp out
3. alter table
4. bcp in
5. create triggers and Indexes.

The bcpout % of space is more less how much of original table size?

Thanks a lot.