Traffic analysis could reveal U-boats without reading Enigma

Would changing German ciphers have prevented Bletchley Park from finding and sinking U-boats?

No; traffic and signals analysis could locate operating patterns from the transmissions themselves, even when their content remained unreadable.

Codebreaking mattered, but German dependence on frequent radio reports generated exploitable information before Enigma traffic was read. Direction finding revealed where individual transmissions originated. Repeated contacts and movement between transmission points allowed analysts to infer a U-boat's operating area from patterns alone. Changing the cipher would therefore leave a major vulnerability intact unless the Kriegsmarine also reduced and disciplined its radio traffic.