Julius is correct, this boils down to the availability of full-stack installation on Solaris.
Have a look at the OneAgent support matrix: https://www.dynatrace.com/support/help/shortlink/o...
As you can see, there are no plans to support full-stack on Solaris. Also note that in "Features" section there is a dedicated entry for "Auto-update of all modules".
Well, the "upgrade" is simple, since you just need to download the latest paas agent archive and unzip it over your current location. That's all unless you modified some of the configuration files such as ruxitagentproc.conf.
The most difficult task is probably just to determine if there is a newer PaaS agent to download. I'm not aware of any documented API endpoint that will give you the list of available agent versions for download.