early potatoes later potatoes

Hullo there fellow gardeners. I was reading in a Alain Titmash book about about early potatoes (he specifically mentions Duke of York, which seems to correspond to Belgian "rode eerstelings" I am planting at the moment). Anyhow, he talks about growing these earlies or possiby using them for maincrop. Does he mean just leave them in the soil longer than would otherwise be the case, storing them or does he mean you have to alter the spacing. He says to space the rows 60 cm apart (although it was 45 cm for earlies in a book he wrote in the 1980s) and 30 cm between plants (30 in the 1980s and currently). Should I put the plants further apart for maincrop? Any ideas on the subject please?

Anton