When the new Diesel Licence was introduced on the Section L licence ( something that has now been given on the Part 66 Licence, check the latest airworthiness notice, they have removed the clause for licenced engineers converting to qualify to certify them)
The company introducing these "New" technology engines in the UK at I think Bournemouth were told there engineers could certify the engines on their Para 6.3 licence for 6 months and at that point they would be required to do an oral and the licence would be issued.........
I say "new" technology because the Zepplins were flying around in 1914 with Diesels installed.