Evening Alfredj,
It can always be dangerous making these type of changes. I just finished the upgrade from 7, 8 to 9 on 5 clusters about 900 Virtual machines. Of those I did not have a single issue with the upgrades. Please don't take this is a go ahead. Please snapshot critical virtual machines before you make the change. The issue you mentioned was an issue with vsphere 5.1 please make sure you have installed the latest update 5.1b and you should be ok and not run into the phanom nic issue. If you do run into the phanom nic issue you can solve it by moving the nic to another port group and back. This can be scripted.
Please let me know if you have any questions.
Thanks,