You buy bureau licences a year at a time. Whenever you buy one if there is a premium licence running at that point you get full credit for up to a year's worth of the premium licence.
So if you upgrade you'll automatically get a full year's premium credit at the point of purchase. So the first year's upgrade (if done before 4/3/2010) will cost £49.90+VAT at current prices. (ie 99.80-49.90).
Once that bureau licence expires a year after it is bought the software will revert to the Premium licence that expires in Mar 2011. If you renew the bureau licence again then you'll once again get credit for the remainder premium licence between 2/12/2010 and 4/3/2011, ie about £15.
Hope that is clear. It is actually very simple and I suspect that my attempted examples may have made things sound more complicated than they actually are. Basically you always get full credit if you upgrade a Premium licence to a Bureau licence and that credit is calculated automatically at the point of making the upgrade.