Will we have to rebuild the integration when the new ERP goes live?

You will have to re-point it, which is not the same as rebuilding it. The forecast models, buffer policies, planning constraints, saved views and planner habits all stay in place, because they sit in the planning layer rather than in the connection. What changes is the source of the same tables: demand history, stock, open orders and master data. That is a configuration and validation exercise measured in days, and it is scheduled work rather than a surprise. Where a data warehouse or data lake already consolidates your data, planning can read from that instead, in which case the ERP swap barely affects the planning setup at all.