Until a common European licence in place, the process has been that when you, as a valid controller from another centre/unit, apply to NATS, you are invited to attend an Assessment of Previous Competence, which normally takes place over a couple of days. This used to be at your (the candidate's) expense.
Must say, there aren't many that come to NATS from Eurocontrol, although there are a few who have been tempted to go from NATS to Eurocontrol.
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