Goal: Change all prices from Euro to Pound.

Following advice given on this thread, I used the following sql patch to update prices on a test site:

Code:
update products set products_price = products_price * '0.83870798';
And then set the default currency to the UK Pound.

This seems to have worked well, although I noticed that an existing 10% off Salemaker was decreased to 7%.

I'd like to understand why this occurred before applying this sql patch to my other sites. To view an example of the incident, please visit this url. I have applied the sql patch to this site.

Thanks for your thoughts on this.