Yes, you can upgrade your device even if there is a remaining installment balance.
However, the number of devices that can be purchased in installments is up to a total of 3 in per communication service contract (= 1 phone number).
You can check the remaining installment balance via WEB de Billing (My au).
For detailed steps, please refer to the following.
Select the phone number you want to check and tap on "Select" [選択].
Enter your 4-digit PIN code and the charges breakdown will be displayed.
Tap on the au phone number in the "au Device charges" [au機器代金] section.
Details such as remaining balance and the number of payments are displayed in the "Installments" [分割支払金] section.
(You will be redirected to the "Individual Credit Purchase Mediation Contract/Individual Item Installment Sale Contract (My au)"([個別信用購入あっせん契約/個品割賦販売契約(My au)] screen.)
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