If I buy a license for Rhino 6 upgrade from Rhino 5 , will I be able to keep running a separate version of my old Rhino 5 64bit , so that I can continue to use T-Splines.
Will the Rhino 6 upgrade work for a student license Rhino 5 version
Yes.
https://www.rhino3d.com/sales/north-america/United_States#tabs-2
Note, you will need to provide proof of student status.
– Dale