There is no mechanism to put the Next Record fields back if you change the merge document type to Letters. It merely becomes a letters merge containing a table. Make that change and then remove the fields from the document.
Start with a label merge, which will add the { Next } fields as the first task, then add your data fields to the first cell. Propagate the data to the remaining cells. Change the document type to Letters, then run the following macro to remove the Next fields:
Code:
Sub RemoveNextFld()
Dim ofld As Field
For Each ofld In ActiveDocument.Fields
If ofld.Type = wdFieldNext Then ofld.Delete
Next ofld
End Sub
Now you can complete the merge.