Insurance is at risk of the Insurer having no liability if there is a causal connection between a breach of warranty or a failure to disclose material facts. Non invalidation protects you from that, as far as that policy allows, if you like it's an extra.
The change in layout, and possible change in the inured sum are material facts. Any breach of the building regulations acops etc, failure to certify electrics gas etc can be breach of warranty.
Best to check the policy as suggested and be prepared to have to act swiftly- the loss of building insurance cover should be of great concern to the tenant and his mortgagee.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com