Read IAS 16.43 "Each part of an item of property, plant and equipment with a cost that is significant in relation to the total cost of the item shall be depreciated separately."
Also note IAS 16.44 " An entity allocates the amount initially recognised in respect of an item of property, plant and equipment to its significant parts and depreciates separately each such part. For example, it may be appropriate to depreciate separately the airframe and engines of an aircraft, whether owned or subject to a finance lease. Similarly, if an entity acquires property, plant and equipment subject to an operating lease in which it is the lessor, it may be appropriate to depreciate separately amounts reflected in the cost of that item that are attributable to favourable or unfavourable lease terms relative to market terms."
Therefore it is allowed to depreciate parts of a fixed asset separately at different rates. However you need to derecognize the carrying amount of the existing seats & recognize the profit/loss of derecognition in profit/loss A/C as per IAS 16.67-72.