I am new to KL being that I just moved to TNL this year.

What are the methods for upgrading on international flights from Y to C?

Is there a certain fare class I need to be in and what fare class will I be upgrading to?

Does KL have visibility on ExpertFlyer?

thanks!

You can upgrade with miles, the chart being available with an online tool at https://www.flyingblue.com/calculator.html
Obviously the higher class the less miles you need.
With KLM you can also bid on an upgrade at
http://www.klm.com/bfu/ub/login

thanks. how do you know if upgrade space is available when you purchase a ticket?

By checking to see if there is a business seat available for outright miles redemption via Flying Blue.

Which is the best use of miles - upgrading from an economy ticket is, overall, a far more expensive way of ending up in that seat.

Is that still the case now that all fare classes are upgradable? It seems you could upgrade from the cheapest economy fare class to C for 72,000 on a Europe-Asia/Pacific. Previously, you'd have to pay a change fee to change the ticket to an upgradable type first, but I don't know if you do now. It would still more expensive but not nearly as bad as it was before, if that's the case.

Yes - outright redemptions are the best (as a Promo Award, even better). What you "save" in the ugprade "fee", you lose by paying for a very-expensive economy ticket (for which there is never a guarantee of getting an upgrade). If you upgrade from a "cheap" economy ticket, you are paying almost as much as if you just redeemed that J seat outright.

Regarding the upgrade with miles, it seems that you need some particular fare buckets to be available in Biz to get the chance to upgrade. Tried to get an M class upgraded with a J4 C4 inventory 5 days before and platinum line said no.
Same seems to apply for upgrade bids, only available if some Z fare still available.

Will try again closer to departure.

You need O availability for most upgrades. And elite/elite+ can upgrade Y to Z.
Agree that award redemptions are generally better value than upgrades but if you are never flexible with dates (thus can't book travel depending on when there is availability), and/or someone else is paying for tickets (and they tend to be Y/B/M/U), day of departure upgrades are an alright use of miles. And in my experience, no particular class needed, just an empty seat.

Obviously, this is a very specific set of criteria that make upgrades sort of worth it for me. But another thing people rarely mention is that award redemptions require all those surcharges which can be pretty high on some itineraries. With upgrades, you spend your miles and that is it.