If its the early tamiya kit, you take the box, put in bin, buy new Panther G. (My 10y/o loved it but even he thought it was lacking)
If Tamiya have any other Panther A kits (I dont know if they do but if they do) find a good reference book and move any things that need to be moved, eg headlights, decide if want zimmerit, many G's dont need it, you may need to change road wheel types, A major difference is the slope of side armor is different (so it sthe same as the jagdpanther, elimination of drivers vision hatch on front glacis replaced with a rotating periscope, hull top hathces were also changed, most G's also had 2 single exhaust tubes, some G'd also had different engine deck layout and late G's also had a modified chin on the mantlet.
hope that helps