If I enter Thailand on a Australian 28 day entry stamp, If I meet all the financial requirements can I get this stamp converted to an O Visa ?

In the process for retirement extension of stay? Yes - you apply with 15 days or more remaining and cost will be 2,000 baht. You must show qualifications for the extension and for retirement that would be the financials available (not needed to be seasoned) and being over age 50

Once again Lopburi, thanks yes so I am 57, have had 800k in a thai bank account for about 2 months

Be aware that not all immigration offices can do the conversion.

I will go down to Jomtien and ask them if they can do otherwise I will get it done in Bangkok

You can do it in Jomtien without a problem.