Can bù (不) change its pronunciation to bú?
The rule about sandhi of 不 is:

It's pronounced in the original tone (the 4th tone) when being used alone or at the end.


规律一:单念或末尾时念本调。
不、就不、毫不、决不、都不、无不、何不、我偏不 


It's pronounced in the 2nd tone when being used before a 4th tone.


规律二：去声前念阳平。
不对、不算、不便、不怕、不必、不屑  不怕、不当、不定、不动、不利、不顺


It's pronounced in the 4th tone when being used before a tone other than the 4th tone.


规律三：非去声前念去声。
不听、不悲、不分、不蒙、不穿、不飞 不同、不曾、不行、 不纯、不回、不白  不美、不想、不好、不许、不准、不满


It's pronounced in the neutral tone when being used in the middle of words.


规律四：夹在词语中间变轻声。
想不开、起不来、行不行、大不了、对不起、大不大、谈不谈、拿不动、好不好、打不开