Wondering if anyone has had this experience. Have a ticket booked PHL-SFO-TPE-CGK. Last leg is booked on Eva air. I specifically booked the flights in W class to apply PP. Turns out that W is basic economy for Eva so can't select seats in advance. Is it possible to just upfare that segment to a non basic fare?

Unfortunately no, the entire ticket will be repriced. EVA may have some fee (vs refaring) to get seat assignments when Basic Economy --- check with EVA or the BR forum

If you booked in W class and it is not BE on UA - and you have applied +Ps to that W ... not sure I see how that would come out as 'BE' on the BR end of things. OP, are you sure this is a BE problem? Many airlines don't allow seat assignments for free. Did you try and call BR? Is this a BR coded flight?

“Basic economy” on EVA does not mean what it does on United. Your fare does not have any particular restrictions.

Furthermore:

(1) you don’t need to book W class to apply PP (although it does decrease the number of PlusPoints required, which I assume is what you meant)
(2) Not all flights booked in W class allow upgrades — you can book Basic Economy and non-changeable fares in W.
(3) On an itinerary with a partner flight number, trying to book online via fare class, if that’s what you did, is not a great idea. In this case it appears to have worked out, because it looks like UA’s W fares to CGK happen to book into BR W class when routing via TPE, but that’s the exception, not the rule. Often, trying to force the partner fare class to align causes the tool to come up with extremely high price points.
(4) if you can find R inventory on the days you want to travel, and you’re not married to BR, you can fly RT In PE (via Tokyo / ANA) for about $100 more than the price I found for a RT W fare ($2600 vs $2500).

BR has taken to calling its deepest discount economy fares “basic.” No free seat assignment until 48 hours prior to departure.

Ahh this is helpful and good to know. I already cleared the upgrade on the SFO-TPE flight into Polaris so not going to rebook on ANA plus my return is from Bali paid about $2000 roundtrip with PP clearing both ways for the overwater flights. Only reason I was trying to adjust the EVA segment is I�m on a separate reservation than my significant other so didn�t want to risk being seated separately. I�m going to try calling Eva and just suck it up and pay the seat fee for both of us.

A wise investment my friend -- after 20 years of partnership I hope your answer is the same!

The dreaded 5-hour TPE-CGK connection! I have done it twice now and it�s a drag. Next trip I am connecting in HKG to Cathay and cutting it to a 4-hour connection. Good luck.

Curious have you ever tried to upgrade that segment at the airport?