@gold rover This is from the DVLA website re online transfers, the link you posted was for applying by post.
The vehicle’s original registration number is usually reassigned to it automatically when you take off a private number.
Apply online
The number will be removed immediately if your vehicle does not need an inspection.
You can
assign the number to another vehicle as soon as you’ve applied to take it off - use the reference number you get after you apply.
There's a slight change of plan, I'm keeping mine for now and just buying his so I can put mine on retention and stick the old plates back on, when I get the V5 he is having his number back as long as he pays the retention fee.