Let's get this cleared up so you can upgrade your line and start enjoying the new phone.
The reason you may need a new SIM card to activate the phone on the line you want is that the new phone may have a SIM card not compatible with the current phone active on the line with the smartphone. If the old and new phone use the same size SIM card then you will not need to get a new SIM card. What smartphone phone do you have on line B, and what smartphone are you looking to upgrade to on line A? I can check if the SIM cards are compatible.
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