Explaining change in PhD project I was enrolled in a PhD program within an experimental topic (experimental physics), but 6 months in the project I had a talk with my supervisor: I was not satisfied with the lack of theoretical work (at my interview we had decided that there should/would be some).

A few meetings followed, where we concluded that it would not be realistic to do any major theoretical work at all. Together with my supervisor we decided that it was best for me to stop my PhD-program and he encouraged me to enroll for a new one in theoretical physics, such that I didn't waste 3 years of my life doing something I would regret. He wrote a recommendation letter for me and I got accepted into a theoretical program. 

Here is my problem: I now have to write a short statement now on why I have discontinued my former topic and started a new one. However, I am worried that it might sound as if I stopped due to conflicts/disagreements with my supervisor, which was not the case at all. Quite the contrary, he was very helpful through the whole process.

Can it be interpreted as such if I write something along the lines of "former project could not accomodate my theoretical ambitions"? How should I tackle this?
Just be direct and honest: you wanted to do more theoretical work in your project and, as you said, you mutually decided a different project and program was in your best interests. There's no reason to try to hide the fact, and mention that your former advisor was fully aware of and supportive of your request. That should satisfy any concerns that you're doing it because you didn't get along with your advisor or because your project failed or anything like that.