I think the problem is those 00000000 values. They aren't convertable to datetime.
Try this
update tablename set newcolumn=CAST(oldcolumn AS DATETIME) WHERE ISDATE(oldcolumn)=1
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability