Welcome to the Psrtutorial Forum, Andjelko90!
In principle, user files from newer models cannot be read by older models. This also applies to larger models, where Registration Banks can be saved as individual files (.rgt).
I'm not aware of any conversion program that can be used to convert an S550 .usr file to the S500 format. The only option would probably be to examine files from both models with a hex editor to find out the differences in the file header. With .rgt Registration Bank files, for example, it is possible in this way to modify files from newer models by a small change in the file header so that they can be read by a certain older model.
Best regards,
Chris