Yes, get a residence certificate from Immigration and ensure you a Thai registered in your name plus take your passport & driving licence (if you have one)
You can apply for a 12 month extension with either 45 or 30 days remaining on your entry stamp, it depends on which Immigration Office you will be using. Do you already have the 400k in a Thai bank account in your name because, if not, it needs to be seasoned for 2 months prior to your application?