Is the change in acctng treatment for office building from the cost model under IAS 16 to the value model under IAS 40 a change in accounting policy?

No. The change in accounting treatment has arisen because of a change in circumstances rather than a change in accounting policy. Entity N's policy for each type of property remains unchanged but the office building in question is accounted for as an investment property from the date of change of use. No retrospective restatement should be made in these circumstances.

Yes, it is a change in accounting policy.

yes departure from one accounting standard to another accounting standard is  a change in accounting policy as stated by IAS8 accounting policies, changes in accounting estimates and errors.