Nick Clegg to make keynote speech at Liberal Democrats Spring conference

The video discusses a tax reform proposal by the Liberal Democrats, aiming to exempt the first 10,000 pounds of income from taxation. This reform is described as fundamental and irreversible, freeing 3.6 million people from paying taxes altogether.

What does the speaker mean by 'fundamental, substantial, and irreversible reform'?

What is the context in which the phrase 'tinkering or tweaking' is used?