Here is another way:
With[{acc = Accumulate@data}, acc[[;; -#]] ~Join~ data[[-# + 1 ;;]] &@Count[acc, _?Positive]]
(* {-1050, -1000, 1, 1950, 1950, 1950, 525, 0} *)
You can also use Tr@Boole@Thread[acc > 0]] instead of Count[...] or if you want ≥0, then you can use Tr@UnitStep[acc].