General Skilled Migration Legislative Changes, Effective Date?

When the Department of Immigration changes the rules for skilled migration in Australia, like MODL (Migrant Occupations in Demand List)- the notices are usually published in the DIMIA website. I am anticipating some changes to the list which may affect my application as yet I do not have all the requirements I need. Does any one know when these changes become effective? If say, they change on 15th February 2009, do the changes become effective immeadiately? Is there a timeframe given to comply - like a certain date in which to comply? Please help.