You sound confident donie, but not sure where you got that.
I flew with an FO yesterday who's been in the company 3 years and joined on a South African ATPL validated by the IAA. He said that validation needed to be renewed every 6 months by the IAA at a cost to him. He's since sat the UK EASA ATPL exams and did the flight test with RYR and is now in possession of the IAA EASA ATPL.
FWIW EASA has only been around for a couple years - far less than a decade
My advice is apply to RYR and you'll find out the actual answer.