my two penneth for what is worth
DVLA Retained Vehicle Identity for "Radically Modified vehicles"
Radically modified can just mean it had a few engines or gearboxes or chassis etc
Each relevant part changed has a value associated with it.
Each vehicle begins with a set value, each change subtracts from that value if DVLA are notified.
If your vehicle reaches you have to re register your vehicle.
DVLA uses a points system to decide what registration number to give a radically altered vehicle.
Keep the original registration number
Your vehicle must have 8 or more points from the table below if you want to keep the original registration number. 5 of these points must come from having the original or new and unmodified chassis, monocoque bodyshell or frame.
PartPoints
Chassis, monocoque bodyshell (body and chassis as one unit) or frame - original or new and unmodified (direct from manufacturer) 5
Suspension (front and back) - original 2
Axles (both) - original 2
Transmission - original 2
Steering assembly - original 2
Engine - original 1
Get a ‘Q’ registration number
You won’t be able to keep your vehicle’s original registration number if one of the following applies:
- it has fewer than 8 points
- it has a second-hand or altered chassis, monocoque bodyshell or frame
- there’s evidence that 2 vehicles have been welded together to form one (ie ‘cut and shut’)
Your vehicle must pass the relevant
type approval test to get a ‘Q’ prefix registration number.
I used to rebuild and restore Triumphs and a few lost their road tax exemption because of this!!!
Plenty of INFO on DVLA website
You should be fine to swap the chassis you speak of, and and just notify dvla of the new chassis no, which you will of course have to generate, we normally use engine no and then first 3 letters of registration. Provided its ends up the log book, stamp away and you are all good.
You could always get a blank chassis plate to make a neat job, and stamp n rivet etc.
Plenty period blanks on the web just depends if you into that level of resto!
To be honest most people don't bother, especially with classics etc.