1) You could return the doors from where you purchased them, I'm sure the company also offers the same doors without the pre-machined door hinges and hole for the door knob. Then you take the measurements from your existing door frame and ask them (store) to machine them according to your specs.

2) Replace the door frame but that would require removing the casing and more work overall. If you are planning in changing the baseboards, casings and mouldings this might be a good solution for you!

Both options comes out to the same cost, new door frame (pine or mdf) will run 10$-15$ or paying for them to machine the doors will also cost you 10$-15$!

Actually a third option would be to patch up the old holes, etc. But I'm not a big fan of this.

Nick