n-channel JFET: importance of gate drain voltage (Vgd) and validity of negative drain source voltage (Vds)
Most JFETs are symmetrical, and the source of an N-channel is defined by convention as the non-gate terminal with the lowest voltage.  You can switch the gate and source terminals and get the same effect.
MOSFETS may be symmetrical, but due to differences in construction meant to improve their performance, most are not.  There may be JFETS which are asymmetrical, but I haven't run across them personally.