Can bailiffs change your locks for unpaid council tax?

No

The enforcement regulations do not provide for bailiffs to break entry to domestic property for the enforcement of unpaid council tax.

In any case, bailiffs may not use force against people.^[1]

Only in prescribed circumstances, bailiffs may enter using reasonable force to enter premises.^[2]

^{[1] Paragraph 24 of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007
[2] See Bailiffs threatened you with breaking entry with a locksmith}