tl;dr: If you had 40 VMs on 8 cores before, you have 10 on 2 cores now. It is the same ratio of VMs:Cores but with stronger processors.
Long version:
"If you take the upgrade, you inherit the new plan specs, vcpus and all.
We’ve greatly reduced the contention on these new machines compared to our old structure, and in testing this new arrangement provides much more consistent CPU time with less potential for steal. We think it’s great and totally worth the move, otherwise we wouldn’t have done it. These machines are incredibly fast, faster procs, SSDs, the network is incredible, etc."
From Caker's comment on the blog. It seems that this was done to reduce fighting over core and provided more consistent fair availability of processing power when they tested it b/t VMs.
Long version: "If you take the upgrade, you inherit the new plan specs, vcpus and all.
We’ve greatly reduced the contention on these new machines compared to our old structure, and in testing this new arrangement provides much more consistent CPU time with less potential for steal. We think it’s great and totally worth the move, otherwise we wouldn’t have done it. These machines are incredibly fast, faster procs, SSDs, the network is incredible, etc."
From Caker's comment on the blog. It seems that this was done to reduce fighting over core and provided more consistent fair availability of processing power when they tested it b/t VMs.