Try converting paid_date to varchar before using substring. Something like this:
select SUBSTRING(convert(varchar(25), paid_date),1,4) + '-' + SUBSTRING(convert(varchar(25), paid_date),5,2) + '-' + SUBSTRING(convert(varchar(25), paid_date),7,2) + ' ' + SUBSTRING(convert(varchar(25), paid_date),11,9)
Mark