1999 Nissan Maxima Will not go into gear when cold

My wife has a Maxima '99 I did all maintenace on the engine, she paid for dealer to do xmission maint. There I belive lies the problem in my opinion but that is another twenty bucks. After noticing the car would not go into gear when the car is cold. It does go into REVERSE even when cold but not DRIVE. The Xmission fluid level is topped and no leaks. The fluild is starting to smell burt and althought I have seam really black xmission fluid it is just not there but close. I changed the transmission filter, reused MOST of the old fluid +plus one NEW Xmission quart. Still have the same problem. I do not know if I should "chance" it and change the entire xmission fluid with new fluid or if this will make things worse -- can you suggest (I have heard that I should change the entire Torque convert amount at ounce).
I know that we are the end of the xmission life (or guessing), however, I am wondering if there is anothing else you would recommend that I have not thought of. Additional information: I do not have any check engine lights on the dash on. When I removed Xmission pan the pan did not have much metal filing just normal high milage minor "slugsh". At writing these I think I should clean out the Xmission lines at the radiator, which I will do. It seams that the problem start slowly withing two weaks before noticing the problem. We are in tight situation and can not afford another car.

Any information or tips is much appreaciated.
Thanks,

Nestor

P.S. After the temperature goes up the shift is normal in RN&D. And in D the Xmission shift at all gears.