Changing Hydralic Fluid over to Conoco Ecoterra Hydraulic Fluid HVI 46

Just purchased an Hitachi EX 300 LC-3 and was wanting to change hydralic oil to Ecoterra Hydraulic Fluid HVI 46 or equivalent zinc free. Not knowing what was previously used in machine how much of the old oil needs to come out? I will drain tank completely but would be better to get remaining oil out of cylinders as well. How is the best way to go about this ? If the old oil does not come out what happens when the 2 mix. After reading some posts on here doesnt sound good mixing them. Thanks for any help !