Hi @user-removed thanks for posting in the Spotify Community!
If you're already paying for a subscription to premium you shouldn't have any problem in other countries, as I understand it there are restrictions on free acounts for periods of time away from home countries but not on premium accounts.
Once you're using a Japanese IP address Spotify should recognise that either way but if you do end up needing to change it you can have a read through this FAQ.
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