If you do that, you end up paying $175 ETF and one month service - but you do get to keep your current number.
On the other hand, if you just upgrade your current line, your ETF in 1 year will be ($175-12*5)=$115 and no extra month of service. But I guess even though you'll save money, you'll have the hassle of losing your old number.
All i have to do is pop my sim out of my 3g and stick it in the 3gs and i wont lose my number. what are you talking about?