Hi, Using CS5 and all is fine under Mac OS Yosemite. Would like to upgrade to El Capitan but I understand that there are issue for InDesign. Apparently, CS5.5 and CS6 work although Adobe say you will not get all the features but won't say which. Prefer not to join the CC due to economic reasons etc.
Question is - if I buying a secondhand copy can I use it and if it had been upgraded by the previous owner to a later version would the serial number and or licence be invalid?
There are countless reports of incompatibility with any version earlier than CC with El Capitan.
If you want to move to El Capitan than you need to be prepared to move to Creative Cloud.
Buying second hand software is dangerous game. Make sure than the seller is fully prepared to fill out the license transfer paperwork.
To more directly answer the licensing question, one part of an upgrade chain cannot be transferred separately -- it's all versions in the chain, or none. Education licenses are not transferable at all.