Writing off a resources does not remove the items from the library catalogue, it only allocates the status of 'W' and a current date to each resource.

If a resource has had a status of 'M' for more than 2 years you would assume that you will never see it again.  Therefore, these resources can now be given the status of 'W'.

ALWAYS backup before you globally update any information.  It is easier to restore from a backup than go through your entire collection and make manual changes.

1. Set the date parameters. The dates that default in are the lowest and highest possible dates.  When changing the higher date range it is a good idea, for continuity reasons, to use a date that has some significance eg 31st December or 30th June and leave the lowest date range. 
2. If you only want to do a section of your collection at a time change the parameters accordingly.