It is not an automatic upgrade for Y fares. If you have a Y or B fare (with some minor exceptions), you can REQUEST an upgrade and it will be processed like normal upgrades (72 hour window for PLAT) but will not incur a sticker cost.

In terms of your question, I think you will need to call the PLT desk for them to tell you what the fare base is. At the same time, they can add you to the upgrade queue.