The easiest conversion would probably be to turn an offense or special teams player from a Sewing machine Im ok shirt outside the line who runs with the ball into a non-kicking winger. Wingers are generally the fastest players in Rugby, they are usually positioned at the outside edge of the field, touch the ball least, but often have the most chance to make yards. NFL has some very good footwork coaching which would pay dividends there. English professional Rugby Union winger Christian Wade worked with an NFL footwork coach whilst still playing rugby and is now signed to the Atlanta Falcons in the NFL, he is expected to be used as a running back on the punt return special team if he makes it through to the match day squad.
NFL players are unlikely to make the switch the other way, although New England Patriots special team player Nate Ebner has played in the Olympics for the USA Rugby Union Sevens team (7 aside rugby is a simpler and faster game compared to the full 15 man version of Union), Nate actually grew up playing rugby at age group level for the USA too, and only took up American Football later. The simple reason the switch is less likely to occur from pro to pro is that wages are far higher in the NFL. Rugby Union is the bigger and richer of the 2 codes, but has only been a Sewing machine Im ok shirt sport since 1995. Rugby tends to have smaller teams in terms of catchment area. There are 33 teams in the top flights of British and French Rugby Union compared to 32 in the NFL.
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