What steps should I take to convert my visa to a Non-Immigrant O visa in Thailand?

You might have a problem opening a bank account without a visa.

This is one of the only things you might need an agent to overcome the problem. I have never used an agent, but it might be worth asking advice from others as I think the cost in Bkk is high compared to other cities, so you need to check their reputation and cost. Good luck.

You first open a bank account and deposit you funds (800k I assume for a retirement visa from an international transfer). You should also in this time visit your local Innigration Office to get their lists of requirements for both a 90 day Non-O (retirement) Visa and the subsequent 12 month Extension of Stay.Then, as Brandon said, when your current entry stamp has a few weeks to run you apply for the initial 90 day Non-O (no seasoned funds required except for Jomtien Immigration). Then, with your seasoned funds and usually 30 days remaining on this Non-O you can apply for the Extension of Stay.

You need to start thinking about it now just to make sure you have time for everything. You'll need to have 14-21 days left on your stamp in order to apply so if you wait too long it could be a problem.