R20 just made some significant changes in the plugin API, so plugins may need to be updated for R20 and create compatibility issues of their own. Some plugins which were working for R19 are no longer available for R20.
COFFEE has been removed from R20, so any scene from R17 that uses COFFEE scripts anywhere will not work.
Fields are new in R20 and will not load with R17. (Which is true for all new stuff in R18, R19, R20 too…)
Render slaves need to be the same version as the main installation; combining R17 and R20 slaves will not work.
And so on… this is just what spontaneously comes to mind. It is a better idea to update the R17 to R20 too; that will save you a lot of grief in the end. Depending on what you want to do with it.
(If you just want a workstation to model stuff on, and move scenes just from the lower to the higher version, R17 will do fine. But that depends on your needs.)