This is would involve racking the beer as a “Delayed racking release”. Our guide on this is at How to rack beer for bottle/cask conditioning without releasing it for sale immediately.
However, if you’re planning on leaving the beer in barrels (vessels), this might not work for you, as you likely don’t want to rack it yet. Additionally, our understanding of the regulations is that this beer needs to be ready to sell before the 1st August, not still in the production process (such as conditing/maturing). As such, even if you do rack the beer with a delayed release, you would need to have actually released it for sale before the 1st August.
If you believe our understanding of the regulations to be incorrect, please let us know (with references to the applicable parts of the Excise notice to support your understanding), and we can review. As it stands currently, Breww will only consider beer for the old duty calculations if it’s fully packaged and released for sale before the 1st of August.
In short: you can’t just mark beer that’s not finished being produced as “Conditioning/Maturing” to apply the old duty calculation method.
I hope this makes sense, but if you need any clarification, please let us know. The new regulations are not as clear as everyone would have liked, to be honest!