I tried to get HP to explain what was covered in their service contract. Was never able to get clarity. The local dealer, from whom I bought the printer (Z3100ps GP 44"), offered a service contract for a bit lower price that seemed to cover what I needed covered: hardware and service costs and software help. I paid for a three year plan. People from this place had put my printer in place and installed the pizza wheel, etc. fix.
So far we have not had to use our service contract. We may end up paying a lot of money for "peace of mind." We shall see. (and the local dealer, as far as I know, has not gone out of business...)
I assume I will have to take care of any software problems. My strategy is simple: be ready to do a complete reinstall of everything, OS and all. Start from scratch, fresh copies of all the new stuff, and roll. My sense is that all HP related software problems have to do with "residuals" of previous versions.
I did not think you could buy a HP service contract after your initial one year warranty expired. I expect it would be impossible to talk to HP and find out whether this was true.