It really depends on whether the existing patina was done using a dark or light shade isn't it?
I would tend to agree (at least as inferred from the term 'doing over') that 'darkening' a lighter patina sounds more achieveable than the reverse.
Although I suspect either way would still involve stripping off the existing patina then re-applying the new patina, in which case stripping off a lighter patina might be easier as well.
Would also suspect that there would be some extent of 'damage' to the leather each time the patina is being stripped off, presumably with the use of fairly strong chemicals/solvents, so that could result in a limit of the number of times one can re-do a patina.