You can't switch from a joint to separate returns after the filing deadline. This is normally April 15, but could be as late as October 15, but only if you filed the extension request before April 15.
(The reason is that when you sign a joint federal return, you are agreeing to take full joint and several liability for all the income and deductions reported on the joint return for both spouses. Once you agree to accept full liability, the IRS won't let you back out. When you file separately, you are only taking liability for your tax facts, and not your spouse's.)
In most cases, you will owe more federal tax by filing separately. It is very rare that you will owe less, and there are one or two common mistakes a tax preparer might make that would create a falsely large refund, and when the IRS catches them, they will recompute the tax and send them a bill. (We can discuss these errors if needed.)
There are some cases where filing separately will cause you to owe more federal tax but less state tax, so you end up with a net benefit of filing separately. This was true of my wife and myself in New York State several years ago, because of differences in where the break points between lower and higher tax brackets were set. But you need to make sure you are following all the rules for filing separately. There are many federal deductions and credits that are reduced or completely disallowed when filing separately.
