must I inform the bank if my shareholders change

Shareholder change: bank must be informed, usually without being asked

Yes, a change in shareholders must be reported to the bank, and usually not only when a questionnaire arrives. The bank does this not out of curiosity: the customer due diligence that banks are required to perform revolves largely around the question of who pulls the strings via the shares. If that changes, those registered as ultimate beneficial owner may also change, and that is precisely the data on which the bank bases its risk assessment.

In practice, this proceeds via two channels that do not automatically coincide. The Trade Register must receive the change within the applicable period — this goes via the Chamber of Commerce, not via the bank. But the bank does not continuously monitor that register; it often expects its own notification, especially if the change is significant enough to affect the UBO registration. If one of the new shareholders is a politically exposed person, that counts extra heavily in the assessment, and the same applies if the new party appears on a sanctions list somewhere. If you do not yet have that information complete, then "not yet known" is a more honest answer than a guess.

Where this follows from: customer due diligence and the trade register

The obligation to conduct customer due diligence is set out in the Anti-Money Laundering and Combating the Financing of Terrorism Act; this act requires the bank to keep the UBO structure up to date and thus to (have) changes to it reported. Apart from this, the Trade Register Act 2007 requires the enterprise itself to report changes to the recorded data — including shareholders for certain legal forms — to the Trade Register. Both notifications exist side by side; one does not replace the other.