Since R-values are cumulative, there is no need to remove what you already have. By layering two different batts together, you get the combined R-value of both batts. For example, two layers of R-19 batts will give you a total of R-38. Be sure to use unfaced R-19, R-25 or R-30 fiber glass batts and lay them cross-wise to the existing insulation covering the joists. Do not put a product with a vapor retarder or facing on top of existing insulation. If there is no insulation in your attic, use R-30 or R-38 full width, faced batts, or fiber glass loose-fill installed to the required R-value.