Do you mean the hanzi pronunciation? Usually not, but sometime they do change according to their compound one. One common example in Mandarin: 大 [da] When that character is standalone, it means big, an adjective. 大船 [da chuan] Big ship but here, the pronunciation changed 大夫 [dai fu] Doctor ( in Chinese 大夫 = 医生 = doctor ) However, the handwritten characters never change. This is because Chinese handwritten system is separated from speaking.

Many characters do have multiple readings. Here are a few examples: 得 is 'de' as a particle and 'děi' as a verb. 觉 is 'jué' in '觉得' but 'jìao' in '睡觉'. 行 is 'xíng' normally but 'háng' in '银行'. 乐 is 'yuè' in '音乐' but 'lè' in '快乐'. 了 is normally 'le' but 'liăo' in了解. 长 is 'chăng' as an adjective but '长' as a verb. You just have to memorise them.

It's stable, but tones can change in some circumstances. Google for 'Sandhi' http://www.sinosplice.com/life/archives/2006/03/29/mandarin-tone-changes

xiexie

any character cant be changed in any case.