In simple terms, when you have finished the work you will need to write to the DVLA informing of the engine change, they will update the vehicle records and logbook to show the new engine number and code.
This does not mean however that you can then just insure the car as a 1.8t as that would not be declaring the engine swap, at the end of the day the car started life as a 1.4 and you cannot hide or get away from that fact.
It is as simple as that, I am not sure why people cannot see this.