On a mobile so can't do any research, but you might get more if you exchange it for an annual from three bridges to crawley or gatwick.
Here's an attempt at a calculation.
Three Bridges to London Zones 1-6 annual season costs £3,776.
Three Bridges to Crawley annual season costs £516.
The difference between the two is 3776-516= £3,260
If you exchange a season for a cheaper one, you are due a refund of the difference, calculated pro-rata to the number of remaining days of validity. This is different from the simple refund calculation, which isn't done pro-rata and is based instead on the cost had you bought the shorter period season on the first place.
So, pro-rata calculations:
Start of validity 30 April 2012
Valid for 365 days (no 29 February within validity period)
If handed in on 6th August, 99 days will have been used.
So,
365-99 = 266 days refunded
266/365 * 3260 = £2,375.70 refunded (no admin fee)
If handed in on 3rd September, 127 days will have been used.
So,
365-127 = 238 days
238/365 * 3260 = £2,125.60 refunded (no admin fee)
So, if the advantage of retaining an Annual Gold Card (34% off all fares after 10am with no minimum fare, and all day weekends) is not attractive to you, in this case then it is better to go for the straight refund. In general, the nearer a season is to the end of its period of validity, the more likely it is to be better value to change over to a really cheap season than to get a straight refund. In this case we are no more than approx. a third of the way through the validity period, and the refund is working out better value.