[Subscription/Change] Can I still use the same number when I change my prepaid service to U+ postpaid service?

Yes, you can use U+ postpaid plan with the same number you used for the prepaid SIM card.
However, for a prepaid SIM card activated with a passport, the same number can be used after changing your personal information from the passport to the Alien Registration Card(ARC) at the existing mobile carrier.