Im not sure about getting the current inserts out, but what you would need to do is get them out, weigh them, then add that weight plus 25 grains and that would give you the correct weight for the new inserts. You could also shoot a 100 grain broadhead and add 50 grains to your insert weight.
Its all be a moot point if you cant get the original inserts out. Food for thought if you ever buy new arrows though.