Changing fare class to be eligible for upgrade?
 

I have some Asia Miles that I would like to use to upgrade from Premium Economy --> Business class on a long haul flight coming up. On the way there, the flight I'm looking at has an R class ticket (eligible for upgrade), but the way back is E class (not eligible). Is there a way to pay the premium to get an R class fare, or do I have to book a different flight entirely?

Phone CX or your travel agent and ask the price of an upfare E to R.
Upgrades are never guaranteed, so it may be a waste of money

I've done this.

But I used KVS Tools to tell me whether or not there's an upgrade available first. When it showed me CS1, I called Marco Polo to see if there was indeed an upgrade available. Sometimes what appears in KVS might no be an upgrade redemption but it could be a full redemption or companion.

Once MP confirmed it was an upgrade redemption, she helped me to change my fare from E to W and then proceeded to issue me the upgrade.

If you're working with a travel agent with a good relationship with CX, they can phone ahead to inquire about upgrade confirmation before processing the E->R ticket upgrade. If not and you don't have any MPC status you might encounter a long wait. Is your upgrade confirmed on the departing leg?