Yes, you can do this but it would be rarely seen in this exam and would need solid justification as to why it is being done in the real world. To change then we would have to show that the new model is more relevant than the old model, which is unlikely. If anything we usually see it go the other way from cost to revaluation model. If you had to do it then there is no specific mention of it in IAS16 so presumably we would revalue at the date of change and then apply the cost model to the remaining useful life.