A datapoint: I tried to upgrade Y -> PE for my flights in May 2024 using Manage My Trip, but the upgrade module is glitchy af.

First, I tried to upgrade my outbound flight, price is EUR 154 for AMS-CGK on Premium Standard fare class. Not bad.
But then I decided not to proceed first, went back and selected the return flight to upgrade as well. Now it costs EUR 250-plus-ish for the same outbound. Weird.
Removed the return flight from my selection, price is EUR 154 again. No seat map was shown and I was brought directly to the Passenger Details page (the one step before payment).
In the Passenger Details page, it kept saying "something went wrong" and referring me to KLM hotline. Any more click on that page brought me back to homepage.
Take 2, take 3, take n... same error.

Second, take 1, tried the upgrade buttom but only for return flight. EUR 204. Same error once getting to the Passenger Details page.
Take 2, now it said no upgrades available for this return flight. Oh well...

I did not try upgrading via KLM App, as there's no button to upgrade on KLM app.

Now I gave "Change Flight" a try by selecting a PE fare for my return flight, both in Manage My Trip online and in KLM app. It costs the same EUR 204.
This time, I got to see a seat map and managed to select my PE seat for EUR 55 minus 30 (offsetting for earlier paid seat reservation).
The next step is... Passenger Details. You probably guessed it right, it's the same error again.
For some, it might better be interpreted that it's not meant to be... or a way for the universe help me keep my 22,900 europenny.

I gave KLM Chat a try and was promptly taken care of by a helpful agent within 5 minutes.
They sent me a payment link (KLM Pay by Link by Adyen) for the same EUR 204 for upgrading my return flight.
I was told to make a seat selection myself after I get the confirmation email, which I received in minutes. Fine...

But... the seat reservation now costs EUR 55 instead of 55-30 = 25.
Started another chat... explained them and gave my seat choice, they managed to offset the price sent another payment link for EUR 25.

Fin.

Anyone knows when KLM offers the upgrade in Manage My Trip?
What are the parameters behind it?

Last time I upgraded from Y to PE, the button was available for both outbound & return flights in both website and KLM app.
Back then, fare classes were V and U (not sure if any of these matters).
Now I've got V and Q, but no option to upgrade yet. And if I click on Change my flight (as I upgraded for one of the legs in the past using this way), it was "not possible to change your flights online".