This Acumatica code will popup a smart panel, prompt for the PO number, and change it on the AR Invoice document.

ARInvoiceEntry_Extension.cs

using PX.Data;
using System;
using PX.Objects.AR;
using System.Collections;

namespace HaunWelding.Customizations
{
  
    public class ARInvoiceEntry_Extension : PXGraphExtension<ARInvoiceEntry> 
    {
        public PXFilter<HWChangePONbrDAC> HWChangePONbrView;


        
        public PXAction<ARInvoice> ChangePONbr;
        [PXUIField(DisplayName = "Change PO Nbr", MapEnableRights = PXCacheRights.Select, MapViewRights = PXCacheRights.Select)]
        [PXUIEnabled(typeof(Where<ARInvoice.status.IsNotEqual<ARDocStatus.closed>.And<ARInvoice.status.IsNotEqual<ARDocStatus.canceled>>>))]
        [PXButton(Category = "Corrections", CommitChanges = true)]
        protected virtual IEnumerable changePONbr(PXAdapter adapter)
        {
            if(HWChangePONbrView.AskExt() == WebDialogResult.OK)
            {
                ARInvoice invoice = Base.Document.Current;
                if (invoice == null)
                    return adapter.Get();
                var Filter = HWChangePONbrView.Current;
                if (Filter == null)
                    return adapter.Get();
                invoice.InvoiceNbr = Filter.NewPONbr;
                Base.Document.Update(invoice);
                Base.Save.Press();
            }

            return adapter.Get();
        }

        public class HWChangePONbrDAC : IBqlTable
        {

            #region NewPONbr 
            [PXString]
            [PXUIField(DisplayName = "New PO Nbr")]
            public virtual String NewPONbr { get; set; }
            public abstract class newPONbr : PX.Data.BQL.BqlString.Field<newPONbr> { }
            #endregion
        }

    }
}