Yes, you can hydroseed over the existing lawn. However, you will need to make sure that the existing lawn is healthy and in good condition before hydroseeding. If the existing lawn is not healthy, then the new seedlings may not be able to survive.

You cannot HydroSeed over the existing lawn. The HydroSeeding process requires that the soil be tilled and prepared before the seed mixture is applied. If you try to HydroSeed over the existing lawn, you will not get good results. You should either remove the existing lawn or prepare the soil properly before applying the HydroSeeding mixture.