Frame Swap, What's the Legal Way?

[windyg]

Many years ago I was in the same position with a bike that was written off due to frame being bent. I managed to get a donor bike of the same model with a v5 but a blown engine. Once I had swapped everthing over I sent off the V5 for the right off to the DVLA telling them it had been scrapped and sent them the V5 for the donor bike telling them it had had an engine swop, 2 weeks later I had the new V5 with all the correct details sorted.