If it was my car, this is what I would do first:
I would start by cleaning the appropriate fuses, that will also serve to reset your TCM. I also would disconnect your battery terminal for 20 minutes for extra measure and let your ECM/TCMrelearn your transmission shifting points. (If for nothing else to rule these steps out.)
If I/you still had a problem... I would take off the center console and clean the natural transmission lock-out switch. (Then let us know how you did this, because I have not done this myself, but I think you can find a YouTube video on it and it's not hard.)
And if you still have shifting issues after doing these electric items... Then I would take out 1 qt of ATF using a $18 Sump Pump on Amazon... and add 1 qt Lucas Transmission Fix. Then drive it for 50-100 miles and see if your tranny shifts better of gets worse.
https://www.amazon.com/gp/product/B083F ... UTF8&psc=1
Note: If you tranny shifts worse don't fret. The goal is to clean out the debris in the solenoid seat. And for some reason, I found our Nissan transmissions don't like Lucas very much. At least my tranny did not like Lucas when it comes to slipping. So you may or may not experience this problem. ...Again, the purpose of using Lucas is to clean and/or to allow a solenoid to re-seat in you transmission valve body and to soften old rubber seals. ...And then you can remove all the Lucas with your old ATF by doing a 100% fluid flush/purge after 50-100 miles or sooner if your transmission starts to work normal; and if not, then no harm, no fowl.
For suggestions on how to do a 100% "purge" you can review this thread for the FX45 (V-8, GEN 1) and adapt it to your V6 model. Specifically, you need to verify which is the ATF fluid return line out of the transmission cooler and be sure you re-fill the same amount of fluid to your transmission if you don't have a dipstick.
Note: This thread is 60% about how to read a GEN-1 dipstick, which may not apply to your car. That said, pay attention to how to "purge" the ATF return line out of your transmission cooler; and through a series of engine starts-and-stops, you want to purge your old ATF out while you add new ATF 3 qts at a time to the tranny ATF-oil pan.
Note: My tranny spec calls for 10.7 qts of ATF, but I found I needed 12qts (4 gal). Just keep track of what you take out so you put back the same amount. And it does not matter if purge 12qts if you put back 12qts.
GEN-1 ATF PURGE TIPS (with a dipstick): topic630322.html
IMO, I do not recommend you let your mechanic talk you into using a transmission exchange pump, which maybe the only way he knows how to do an ATF flush. So you may have to "educate him." Or DIY and save some money! The process is easy once you understand it!
Note: If you put Lucas in your tranny, and you want to get it all out, including all the ATF in the torque converter, then do the "purge" like I recommend. Why? Becuase all these ATF exchange pumps dilute your old ATF with new ATF and that's not as good as getting 100% of the old ATF out!
The above is a DIY approach to your problem. The other way to go is to let a professional transmission shop do some tranny solenoid probing and prepare yourself for a $800 diagnosis with no guarantees they will just tell you you need a transmission overhaul. So when it only costs $20/gal (x4) for Valvoline Max Life ATF, which is compatible with your Nissan Matic "S" ATF, I would go this route before I go tot he tranny shop, and if he can save your tranny you an always reuse your Valvoline Max Life ATF.
Note: If your tranny uses a metal screen for a filter, like mine does, then I don't recommend you change it, because metal screens do not degrade over time. And it's more work you don't need to do.
Again... verify your transmission cooler flow. This diagram is for GEN1 FX35 and FX45 models. ...Notice the cooler does not care which way the ATF flows. I.e., it's the same cooler. Only the input and output lines are switched depending on the model. Moreover, you might like to know the rubber return lines are very easy to remove and reattach to the transmission cooler, which makes this fluid purge process easy! You just need 2 people!