The way to do it would be to sit in the RHS of a 400 for 7 years until senior enough to get LHS A320. The only reason you'd go to the RHS of an Airbus from the Jumbo would be to get used to the Airbus/Shorthaul pre-command. However there is no requirement and people have, do and will just go RHS 747to LHS Airbus. It's all about seniority numbers. If you have the seniority, aren't frozen and there are vacancies you will get what you bid for.