Breww will calculate the duty on any beer moving past the duty point in the same way you’ve described here. So anything that leaves duty suspense before the 1st of August will use the current duty regime and anything that leaves after the 1st of August will use the new regime, regardless of when that beer was produced. If you are moving stock in bulk between bonded and non-bonded locations, then a stock take is currently the most efficient way to record this transfer within Breww.
However, I should mention that if these products haven’t passed the duty point and are actually still duty-suspended, I’m afraid we can’t advise on whether you’re allowed to simply declare that you’d like to pay the duty on your existing high-strength beer stock whilst it is still in duty-suspense. You would need to check whether that’s possible with HMRC.