Per the cert terms...
So if you book Delta-marketed flights, you can book any fare class except E, and upgrade into KLM Z class. If you book KLM marketed tickets, you need Y/B/M.
I've never found any difference between Z inventory on the KL flight numbers and the DL codeshare. If you get an availability display from KLM, it returns Z inventory for both the KLM and Delta codeshare flight (DL9667/KL624 in the example below)... I've never seen a difference in availability between the KL flight number and the codeshare.
Code:
115DECATLAMS-Z¤KL«
15DEC THU ATL/EST AMS/‡6
KL RESPONSE ** DIRECT CONNECT PARTICIPANT **
** KLM ROYAL DUTCH AIRLINES - AN ** 36 TH 15DEC 0000
7KL 624 Z5 ATLAMS 400P 600A‡1 772 0
8DL*9667 Z5 ATLAMS 400P 600A‡1 772 0
9DL 72 Z0 ATLAMS 552P 815A‡1 333 0
10DL 74 Z0 ATLAMS 810P 1035A‡1 333 0
11DL*9375 Z0 ATLAMS 1025P 1225P‡1 772 0
12AF 689 Z1 ATLCDG 850P 1050A‡1 772 0
13AF 1640 Z1 AMS 1150A 105P 318 0
14AF 681 Z1 ATLCDG 535P 740A‡1 77W 0
15AF*8224 Z1 AMS 910A 1030A 73W 0
16KL*7206 Z2 ATLDTW 1235P 238P 757 0
17KL*6018 Z2 AMS 359P 540A‡1 333 0.
If you request the availability display from Delta instead, it doesn't show the KLM flight numbers only the Delta codeshares... but again, the inventory is identical.
Code:
115DECATLAMS-Z¤DL«
15DEC THU ATL/EST AMS/‡6
DL RESPONSE ** DIRECT CONNECT PARTICIPANT **
7DL9667 Z5 ATLAMS 400P 600A‡1 777 0
8DL8672 Z1 ATLCDG 850P 1050A‡1 772 0
9DL8531 Z1 AMS 1150A 105P 318 0
10DL 95 Z2 ATLDTW 1235P 238P 757 0
11DL 132 Z2 AMS 359P 540A‡1 333 0
12DL8672 Z1 ATLCDG 850P 1050A‡1 772 0
13DL8537 Z1 AMS 1240P 155P 321 0.
In this circumstance, you should be able to buy a ticket on Delta for travel on DL9667 and upgrade that into Business class using a GUC. But Delta's IT apparently tries to show OP inventory for these flights, so agents will try to tell you there are no upgrades available.