Of course there are situations or sentences in which 'but' and 'however' are interchangeable without a significant change of meaning - but that's a different question from whether two words can be called 'interchangeable'.
In a certain context, "Get that animal outside!" and "Get that dog outside!" have the same meaning. But that doesn't mean that 'animal' and 'dog' can be considered interchangeable.
Interested in Language