Hi
It depends really on how the buns tums cope when you are changing. For my buns I tend to change over a 5 day period, doing a quarter, then a half etc cos I know their yummies will be Ok. If you want to err on side of caution you could do it over 10 days, 5g of the new stuff per day. So first day is 45g of Excel, 5g of SS. Second day is 40g of Excel, 10g of SS etc. Obviously if you get any upset tums then take it slower than this but I'm sure this should be fine. Do you give the same amount of SS as you do Excel, or do they recommend giving a different daily amount?
Hope the change over is successful!
Nicola