Yes, you can upgrade your device even if there is remaining installment amount.
However, the number of devices that can be purchased in installments is up to a total of 3 devices per communication service contract (= 1 phone number).
The remaining installment amount can be checked in the WEB de Billing on My au (Web).
Please see below for how to check the remaining installment amount.
Select the phone number you want to check and tap on "Select" [選択].
Enter your PIN and tap on "Next" [次へ].
The screen showing the charges breakdown will be displayed.
Tap the au phone number in the "au Device Charges" [au機器代金] section.
Details such as the balance and number of payments will be displayed under "Installment Payment" [分割支払金].
(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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