In complex behaviors like speech and dance, the serial order of vocal syllables and motor gestures is governed by syntax rules. Canary song is a variable sequential behavior composed of subsequences of syllable repeats, called phrases. These phrases are flexibly ordered to form a large repertoire of songs, following long-range syntax rules, which relate the outcome of a phrase transition to the identity and order of preceding phrases. Projection neurons (PNs) in the canary premotor brain nucleus HVC show syllable-locked activity patterns which indicate their part in syllable-driving premotor 'neural states'. Still, the dependence of phrase transitions on this activity remains unclear. A commonly held model hypothesizes that HVC premotor states are binary, either on or off, deeming transitions independent of past context once a state is active. Here, we recorded HVC in singing canaries and tracked individual PNs across weeks. This large dataset allowed us to examine each cell's activity in its preferred phrase type in multiple songs and to show that the downstream phrase transition depends on this activity level, indicating that neural states are not all-or-none events. We then observed that canaries change their phrase transition probabilities across days. We show that this dynamics is not fully captured by changes in the pre-transition neural states - indicating a possible plasticity of the neural mechanisms for transitions between states. These findings suggest that birdsong syllables, and their underlying neural states, may be less stereotyped than previously believed and that state transitions may be plastic across days and weeks - affording a new model for brain mechanisms that underlie plasticity of syntax rules in learning and in new social or environmental conditions.