Yeah, no consequences of doing that, right? Like all those paid apps who suddenly get crowded out of their "top paid" positions to some $0 app that gets downloaded 10x more? I guess if the rankings are based on $ amount (they aren't that's why the "top grossing" category was created) then it might be fair.
It seems like the only "fair" alternative is to have a 3rd category, but as the developer of a "paid app" with currently no free apps, I personally don't think that leaving them in the free category is going to change much in terms of ranking, reviews, etc. but maybe that's just my bias...