I don't really see how Z-Bo is better than Horford. They're equal, if anything.
Z-Bo gets 2 more rebounds, but that's really it. Scoring is more/less a wash, as are steals, Horford gets 2 more assists, more blocks, fewer TOs, better FG%. Horford's FT% is low currently, but it'll get back to its career norm of around 75%.
The only reason I'd do this is because I need a little bit more rebounding.