me thinks you're making it too complicated
Assuming you have Re-registered the plane (it is not allowed just to paint any Reg you like on it - you have to have a CofR) then you simply remove the old one by an Approved process (sanding is probably best as you'll on be removing the "top coat") check the rules on size and placement for whatever country the New Registration is for - paint using an Approved Paint.
I am low on AI stuff with me (just spent two years transferring B733's) but you should all the info you really need in SRM chapter 51.
The CML (Consumable Material List) should tell what paints you can use on your plane.
Normally we paint the new reg on before the weighing or use 'sticky letters' and let the new operator paint it on in his shop - but the actual change in W&B is insignificant so few people ever worry about it.
Don't forget you are REMOVING the Old so unless you are using far bigger letters a long distance from where the old one was you're not adding much weight (in fact you may be reducing it) - but if you are really worried simply weigh the paint, paint it on and weigh what you have left - the difference is what you have used - crunch the numbers for each location and you'll probably find a trolleydolley serving the pilot with a coffee moves the CofG more.
You can do it all on a Jobcard - it's probably better if you do an Engineering Order with SRM CML as references to Approved Data.
Hope this helps
Remember do Not let the Authority see the new reg until after they have issued the new Certificate of Registration - they almost all get touchy about this sort of thing.