What OS are you upgrading to?
Cubase 9 is qualified to run on 10.11 and 10.12 OS Cubase 9 Updates and Downloads | Steinberg
So you can only upgrade your os to 10.12 accord to Steinberg.
SKYPE FOR MAC:
Mac
Version
Skype on Mac requires Mac OS X 10.10 or higher
Processor
At least 1 GHz Intel processor (Core 2 Duo)
RAM
At least 1 GB
Additional software
The latest version of QuickTime