We experimentally demonstrate low-cost and low-loss conversion of OM3 to OM4 MMFs using strong mode mixing by simple LPGs for datacenter applications. OM3 MMFs can be converted to OM4 MMFs (error-free transmission at 10 Gb/s over 400 m) with only 0.2 dB loss. Error-free transmission can be extended to 530 m with a larger loss. With suitably designed index profiles, the proposed method's fabrication tolerance exceeds that for OM4 MMFs.