As Jaylou said you can change your target whenever you want while you are losing.
I e-mailed SW asking if I had to lose 7 pounds if I changed my target once I reached initial target, and this is what they told me:
Thank you for your email received at Head Office. If you are currently within our healthy BMI range and are wishing to tweak your Target then you can do this by less than 7lbs Sonia, but if you have taken Target outside of this healthy range than it must be amended by at least 7lbs.
I have changed my target twice since I reaced target in December. I went from 9 1/2 stone to 9 stone 2.5 pounds and last week sent it to 9 stone, though I was 1.5 pounds below that.