Following up - I guess you guys may be interested.
So I went to upgrade our production server last night (reverted it back to the old version, but will do the procedure again soon enough).
So pretty much everything worked out-of-the box, besides two major things.
- ESXi hosts multipath is renamed (not gone or anything - renamed). If the monitoring service with 1.5 is called something like “Multipath L20 physical”, with Check_MK 1.6, the same thing is discovered with a WWN number. So this is some manual work (the ESXi hosts are 41), but it is OK.
- A lot (and I mean a lot) of the Oracle checks are gone… the discovery cannot find them. Checks like ASM groups getting UKNOWN status, Inventory jobs getting CRIT status…
- So what I suggested to do is to add a Oracle host (srv1 let’s say) to the already updated test Check_MK server. Then ask the Oracle people to check if what they “see” there is enough for them.
- Then, as a follow up, I am thinking of updating the Check_MK agent on the srv1, and see what checks will appear/disapper…
Basically - Oracle is fancy and makes problems 
P.S. I guess there is no problem for a certain VM to be monitored from two Check_MKs ? I understand that when/if I update the agent of the srv1.