[SOLVED]Need to change the layout of the integrated reply emails

I'm currently preparing the new look of my e-shop based on the new LC 2.6 (dev) and have already adapted my template. However, I also need to change the layout of the integrated (order / confirmations / ...) reply emails. Before I continue to change a PHP code that is still in the developing process and being changed, I'm asking if there is another solution. I have no idea when the final version 2.6 will be released and I do not want to rework the PHP code of these replies, which will then certainly be overwritten with the official 2.6. 
Are there any suggestions?

I hear this article could be good reading for you. :)
https://www.litecart.net/en/wiki/how_to_prevent_losing_changes_before_upgrade

I'm not sure what your question is?

2.6.0 could be days before going master.

Thanks Tim! I've been using the Winmerge process for a long time, especially to keep my custom templates up to date. 
Excellent! You answered me by announcing the release of the final version 2.6 soon. So I'm waiting for this final step and only then will I proceed with optimizing the email code. Thank you very much!