Verizon is / was deploying the Ericsson 5GTF backend systems which is their proprietary 5G pre NR standards setup.
The M-mimo equipment on cell towers / light poles are dual mode 5GTF and NR capable. So it's fairly simple for them to just "flip the switch" (simple terms).
They had built a completely new standalone system for 5GTF that doesn't really interact with their extensive LTE EPC hence why they can go full stack NR rather than mixed which most carriers will be going with.
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