Professor wants to change my thesis' interpretation in favor of his research. How to react?
It sounds like there may be potential arguments in favour of both interpretations. First pin down what they are - then you're in a better position to have a discussion with your professor. It may turn out that one of you is unambiguously right - and that's a good way of finding out. More likely, there are arguments for both interpretations, in which case you might be best off presenting both together with some critical analysis.

TL/DR: Follow the evidence. Without providing any evidence, neither of you is demonstrably right.