[R] xts object translation

[Michela]

i have an xts object:

                      aaa               bbb            ccc              ddd
2001-01-02     179.7061        239.11      1712.60         271.10
2001-01-03     181.1751        243.24      1689.10         267.15
2001-01-04     181.6126        242.09      1737.86         267.10

i simply want translate upward all the values
So values in first row are lost. All others traslate upward

In my example, xts object should become:

                      aaa              bbb           ccc             ddd
2001-01-02     181.1751      243.24     1689.10        267.15
2001-01-03     181.6126      242.09     1737.86        267.10
2001-01-04     NA               NA           NA               NA

Any idea how to do this upward translation?

thanks