Paris to nice

A multiple destination search function will show how to fly into Paris and back to Boston on a single itinerary. Or vice versa. It will be about the same price when you subtract the cost of returning to your arrival city. It certainly saves time, even with a flight connection between Nice and the trans-Atlantic ride. The search function will line all of that up for you. I prefer enduring the longer route first and then returning from London, in part because I don't have get up early in the morning for the return. Meanwhile, the one-way ride by fast train between the cities is interesting -- once.

You CAN change your flights plans. That would mean changing from round trip tickets to a multicity (NOT 2 one way) tickets. Ahd it's often a time saver. Whether you SHOULD do it depends on your airline, the type of ticket you now have (what change restrictions are on those tickets) , and the costs to make that change.

Perhaps, or take the high speed train.
https://www.thetrainline.com/en-us/train-times/paris-to-nice

Because the tracks between Marseille and Nice are not made for fast trains, there’s no such thing as a high-speed train between Paris and Nice. It’s six hours plus another hour to get to the airport.

I was just going to say what Betsy said.

Yes you should see if a change is possible and what are the restrictions. The flights from NCE are normally not more expensive than returning to Paris first, you will just have an additional connection (often through a less trafficked city), although the presence of the discount carriers starting up again may change that.

Since the train down & back takes quite a bit of one day, I would do it if you have an extra 5 days in your itinerary to devote to Nice & the surrounding area. I did that on my trip in 2019 (along with other parts of France). There’s so much to do in the Nice area that even having the four full days seemed rushed. The train was an easy way to travel - just takes a day from your other options to be out enjoying yourself.