As you can see in burials of Norsemen everywhere, they were sometimes buried with both a cross and a hammer. They were very open minded, and 'conversion' to them did not mean abandoning their Pagan religion. It was only later in the viking era when Christians started to kill Pagans who would not abandon their Pagan religion, that Pagans refused to have two religions. But with the exception of those situations, conversion generally meant that you would still be a Pagan, but just also a Christian - in one sense you were being smart because you would have 'double' the protection. I think if any conversion from Christianity to Paganism was to be had, it would be Norsemen who married British people, whose children were educated in both of the religions, and cherished and worshipped both Christianity and Paganism. The TL;DR is basically that the Norsemen were too tolerant to just have 'one' religion, so the conventional use of 'conversion' doesn't always apply to them when they adopted Christianity.