This paper presents a method taking into account the form of a tune upon several levels of organisation to guide music generation processes to match this structure. We first show how a phrase structure grammar can represent a hierarchical analysis of chord progressions and be used to create multi-level progressions. We then explain how to exploit this multi-level structure of a tune for music generation and how it enriches the possibilities of guided machine improvisation. We illustrate our method on a prominent jazz chord progression called 'rhythm changes'. After creating a phrase structure grammar for 'rhythm changes' with a professional musician, the terminals of this grammar are automatically learnt on a corpus. Then, we generate melodic improvisations guided by multi-level progressions created by the grammar. The results show the potential of our method to ensure the consistency of the improvisation regarding the global form of the tune, and how the knowledge of a corpus of chord progressions sharing the same hierarchical organisation can extend the possibilities of music generation.