Reuben is on a Full season Standard Loan with Argyle.
According to Football League Regulations (Players) paragraph 53 2.7 full season standard loans can become permanent transfers at any time during the period of the standard loan (which expires at the end of the season, whenever that may be)
It would appear, therefore, there is no regulatory impediment to securing Reuben on a permanent contract before the end of the season, rather than waiting for 1st July 2014. That would be a statement of intent for next season.
http://www.football-league.co.uk/regula ... 33_2125731