Transaxle oil change at 135k?

Better late than never.

The Prius doesn't have a CVT in the manner you're probably thinking. It's an eCVT which operates completely differently. Put very simply, it uses two electric motors and a planetary gear set to distribute and proportion power to and from the wheels, engine, and battery. No clutches (except limited travel damper plates connecting the engine & transaxle), and no torque converters. It's a marvel of engineering.
If I'm not mistaken, the v has the same transaxle as the regular Gen 3 Prius which is a P410. Here's a quick look at how it works. Professor Kelly isn't the most exciting teacher, but his content is tremendous.

 

In my experience a drain and refill causes no harm.
A power flush can cause problems.

I would be inclined to do the oil change.
Good luck with your project.

 

Change it. You electric motors will thank you.

 

Nothing wrong with changing it now. 125k sooner would be even better.

 

Stick with Toyota ATF WS fluid. Purchase 4 quarts (plus the fill and drain bolt washers).

Torque for both bolts is 29 ft lbs. Remove fill bolt first, just in case you encounter problems. A longer handled wrench or breaker bar is good. They’re both 10 mm socket head cap screw, and there are Allen-key style “sockets” available for ratchet wrenches. Don’t even entertain using an Allen-key and pipe extension.

Correct level is easily achieved by filling till it flows back out, with the car raised and LEVEL.

It’ll take about 3.5 qts to fill. A funnel with hose from above is dead simple: just go slow, 100 cc at a time, beyond 3 qts. Toyota recommends to NOT save left over, just recycle.

Addendum: Attached Repair Manual excerpt has transaxle fluid instruction. It's for regular 3rd gen, but the same procedure for the v:

 

Also, it's wise to note the steps in the instructions: loosen the fill bolt before the drain bolt just in case you have problems getting the fill bolt loose. No fun draining the fluid and then discovering you can't fill it because the fill plug is goobered up.