IAS 16.53 - "The depreciable amount of an asset is determined after deducting its residual value. In practice, the residual value of an asset is often insignificant and therefore immaterial in the calculation of the depreciable amount."
If you have not considered residual value by error then account for as correcting an accounting error (ie. retrospectively).
If you considered that residual value is zero before and now you think that there is a significant residual value, then account for as a change in accounting estimate (ie. prospectively).