The only way I see this having a snowball's chance on Mustafar of working is thus:
1) The initial star level of the character determines the value per shard
2) Credits are used as part of the exchange (akin to changing a mod).
Thus the lowly CUP can't be a 1:1 for GK. I can foresee a 5* shard being worth 10 4* shards or 20 3* shards or 40 2* shards or 80 1* shards. So 80 CUP shards equals one GK shard (or 2 Jyn shards or 4 Chirrut/Baze shards).