I second Regan’s advice! I started with non-GS1 and saw the writing on the wall so switched to GS1 a couple of years ago including my back catalogue – it was painful to make this change in Amazon’s system and I would not advise attempting unless you are forced to! As Regan says if you have not used the 5 old codes yet then bin them and suck up the GS1 cost, it will save you a massive headache! If they are already in circulation then at least my experience gives you hope that if ‘the sleeping bear’ wakes it is possible to unpick this however its just not fun! Avoid avoid avoid!!