The short answer
Separate the service agreement from device financing, promotional credits, and number-transfer requirements before switching.
Step by step
Read the latest bill and account agreement to identify device balances, contract terms, promotion end dates, and any final-bill rules.
Ask the current carrier for the exact payoff amount and whether leaving will forfeit future device or trade-in credits.
Confirm that every phone is unlocked and compatible with the new carrier before ordering service.
Start the number transfer with the new carrier while the old line is active; do not cancel a number you want to keep.
After every number works on the new service, check the old account for remaining lines, watches, tablets, autopay, and the final bill.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.