Let me help you for free. It depend. Typically the law immediately applies from the day it's gazetted. In some law - they can specify the start data when it becomes effective. On matters already before the court - esp criminal - they don't apply retroactively NEGATIVELY.
The KEY CONSIDERATION for retroactivity - is the EFFECT of the law if something was already in dispute - for instance if the law increase punishment for some crime and you already comitted that crime - then it doesn't apply - you still get the lesser sentence. If the law has positive effect on your case - say it declares smoking weed legal - then it applies immediately - and you get discharged the same day.
Bottom-line- any law that seem limit rights and freedoms of individual cannot apply retroactively.
If you're talking about election amendments - this is fresh election ordered - and the new laws will apply. This is essentially a civil matter.
and how mortals can be protected from the same. When does the principle kick in.