Is it advisable to change transmission fluid at 140k?

Change the fluid. Conventional transmission advice does not apply inside a Prius transaxle that has 650v three phase and no conventional clutches. Also don't allow any other fluid except Toyota's special transaxle oil. Same reason, it is used as an insulating and cooling fluid.

 

Yep go ahead and drain and fill...no need to do any "flush". Since the bolts wouldn't come off that probably means it's never been done so expect the old fluid to be pretty black when it comes out.
For folks who want to keep their cars a long time, the suggested change interval is every 6-year or 60,000 miles...whatever comes first.
To get the old bolt off, spray a little penetrating oil all around the bolts and let it sit for an hour or so...then you can use a breaker bar. You can make one yourself with just a metal pipe that's about a foot to 18 inches long....just put it over your ratchet and turn using the bottom of the pipe. I've, also, seen at Harbor Freight extra long ratchets for 3/8 and 1/4 inch sockets that does the same thing.
Best of luck! (Don't forget to buy a few crush washers!!)

 

My father-in-law was always harping about "you've gotta have the right tools". Case in point:

 

Yep... Primary difference between DIY and taking it to a shop is a good shop is constantly restocking commonly used parts and optimizing techniques with better tools.

 

It helps also, to have a higher-lift floor jack, and taller/beefier safety stands, to give you more elbow room to use a long-handled wrench. What's a strain with a regular handle socket wrench becomes easy, with another foot of leverage.

Or use an impact.

 

Open the fill bolt first.

You’re up a creek if you open the drain bolt and then learn you can’t open the fill bolt.

 

$40 is reasonable.

 

It's advisable to change it every 50,000. No need to be a fanatic about it. Next time you'll change it after this is at 200,000. Change the oil every 5,000. This makes it easy to remember when it was done last

 

Your Prius will love you lol.

 

Yep that's one of my favorite to break stubborn bolts! I just drain/filled the transmission on my daughters' 2012 Sienna and had to use it but need to be careful...when those bolts break free they are free, indeed, and hitting your knuckles on the concrete is not a good feeling!
Her Sienna surprised me, it's a V-6 and I only got 2 quarts of transmission fluid out...so that's what I put in. My Tundra (V-8) got over 6 quarts out.

 

Geez, for the Sienna I might splurge for a short tech stream subscription: see what they say, regarding fluid volume and changing procedure. That said, I appreciate conventional automatic transmissions typically only drain about 50%.

 

I think it was only a little lower than expected...on my Amsoil account it has a convenient capacities section and only lists 2.9 Qts for the Sienna. But it was pretty black, not sure when it was done last (she bought it used a few years ago) so I'll do another drain and fill next summer for her.
FWIW, setting up an Amsoil account is free and lists our lubricants and fluids our vehicles need, capacities, and even torques for various drain/fill plugs.