I don't do anything fancy like tax preparation or tracking of stocks.
When I upgrade (to El Capitan or High Sierra) I do not want to suddenly lose my capacity for function in Quicken Essentials.
Quicken Essentials will apparently function in El Capitan and not in High Sierra.
I have by now concluded that my only obstacle to High Sierra upgrade would be Quicken Essentials.
My only reason to want to pursue High Sierra rather than El Capitan is so as to be more up-to-date - especially security-wise.

Could I somehow divide my computer space to have two operating systems, my current Mavericks and High Sierra?
That would enable me to upgrade, generally operate under High Sierra and for Quicken Essentials jump back to Mavericks.
If I could do it that way, I would be able at a later time of my choice ( e.g.beginning of a year) to explore what other basic system might be a good substitute for Quicken Essentials so that I would be able to drop the Mavericks OSX.
One concern in connection with this idea of mine:
Would it negate the added High Sierra security on my computer if I retain a more vulnerable Mavericks (even if I would no longer use that OSX for internet or e-mail)???