Hi
There is No such thing as a "Silly Question".
And just for information, HP Z420 Workstation has reached "End-of-Service life" (EOSL) in June, 2016.
And Z420 is a very efficient machine and for all the information related to HP Z420 Workstation, below link will be very helpful.
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04112787.pdf
For Windows 10 upgrade you will need “the latest genuine version either Windows 7 with Service Pack 1 (SP1) or Windows 8.1.
For more information on HP Z Workstations, Please refer to below link:
http://www8.hp.com/us/en/workstations/z240.html
I hope that answers your question. 
I am an HP employee, the opinions expressed here are my personal opinions, not of HP.
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