You can open any PLECS circuit in a newer version of PLECS. If you have a schematic created with PLECS 3.1, just open it in PLECS 4.1.
The other way round is a bit more complicated. The latest patch version z of PLECS x.y.z can open a schematic created with PLECS x.y+1. If you have a circuit created in PLECS 4.1 you'd first have to open it in PLECS 4.0.8, save it, open it in PLECS 3.7.5, save it, open it in PLECS 3.6.9, save it etc. This assumes that you have only used components that were already available in the earlier PLECS version.