There are multiple outcomes for that depending on whether or not you can use platinum cross-platform or if you are required to use the platinum in the original platform it was obtained from.
Nope. I can see them making platinum stay on the platform you purchased it on because otherwise people will extremely large amounts of platinum could move them between platforms and cause problems or you can have platinum on the platform but you can't use it for player to player purchases or something along those lines.
Cross-Play & Cross-Save Megathread - The Soonening(TM)/Public Test Edition
in General Discussion
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Nope. I can see them making platinum stay on the platform you purchased it on because otherwise people will extremely large amounts of platinum could move them between platforms and cause problems or you can have platinum on the platform but you can't use it for player to player purchases or something along those lines.