No such thing as a straight swap ,when your changing radiator sizes.
Depths are different for pipe tails ,unless you don't mind crooked pipes .Brackets are usually different ,tall rads should be flushed through as well.
As already mentioned your pressure relief/temp valve from your cylinder is not connected correctly,(photo one) it should go to a tundish then be piped outside to a drain .Yours looks like it goes back into your heating system.
Ask him if it passes inspection .
My BI would not pass my extension until a similar 'mishap' had been corrected .
I had to cut the roof back past the firing and reinstate with the correct fall.