Well, its almost always best to keep the amount you lower a car the same front and rear, though lowering the front more is sometimes done for aerodynamics. Lowering the rear more and as a result raising the front is generally not a great idea as it lets air pressure build up under the front end and reduces steering control at higher speeds. Plus it tends to look strange.
I think I would be tempted to try fixing this as a two stage project, first putting lowering springs in the front if you find in your research on them that they will lower the front to at least an amount equal to what happened with the replacement springs in the back. Then you could see if you like the handling of the car with the mixed spring situation before spending more to replace the backs with lowering springs also, which you would only then have to do if the result left those rear springs feeling too soft.
However, if checking how much lowering you get with fronts does not find they will give equal or more lowering than happened in the rear, I think I would just replace all four, as no matter what, you really do not want the front end higher than the rear.
pax, smn
