At my unit there are no specific local instructions covering this but generally perceived as:
Go-around = aircraft has commenced descent from normal platform on the GDP
Reposition = any point previous to above (e.g. aircraft on ILS LOC but has not yet intercepted the GDP)
(Practically speaking - from ATC perspective at my unit - a go-around requires paperwork, a "reposition" does not necessarily require paperwork!)
Hope that helps