I certainly wouldn't want to try this.
My van was uprated from 3500kg to 3850kg by process of replating only. (it's based on a Fiat Maxi, and as stated elsewhere, with appropriate tyres and springing supplied as part of the Camping Car package its design weight is 3850kg, plated down to 3500kg to avoid numerous restrictions).
The plate was provided by SVtech, who also provided certification to a standard required by the Vehicle Inspectorate.
As far as I know, if the work is not carried out to the Vehicle Inspectorate's standards (which includes a qualified engineers assessment and certification that design limits have not been exceeded), then replating is likely to be seen as illegal.
Somewhat more relevant than this, unless you can provide evidence of the process being underwriiten by a qualified person, your insurance is likely to be invalid (likewise, you should inform your insurance company of any change).
Part of the fee you pay provides you with professional advice on safety and the design limits, and protects you against any of the above, since if necessary you can claim against the certifying body.
Every vehicle I have ever owned has had a combined axle rating greater than the GVW. Blithely assuming you can uprate under all circumstances above the GVW but below the combined axle rating is IMO somewhat foolish, since in many circumstances this may exceed the safe load for braking capacity.
In addition, uprating above 3500kg places the vehicle in a different taxation class, requiring amendments to the V5, additional road tax, and certain additional restrictions, both in use and who can drive.