You can upgrade to Business without paying a change fee (usually) by asking nicely or sometimes it lets you do it when you select your seats online. It will be expensive as this is just putting what you currently paid towards the lowest J fare available for your flights.

The other option is to buy up to Y/B/M, which you also can usually do without paying a change fee if you ask nicely, YMMV. Then you can use miles to upgrade.

Finally, as others have said, KLM has a rep for offering check-in upgrades when there is space available.

I agree, Delta rules?