I've used Michael Bedesem's Midiplayer utility to make older files play on newer psr's. I think it can fix some issues and it also has a batch convers1on mode.

It's here http://michael-bedesem.software.informer.com/

As to sysex, maybe you need to add a GM or XG reset at the start of the file? Having said that I just looked at the psr950 data list and there's no sign of xg reset. There's few GM codes such as F0 7E XN 09 01 F7 which is GM1 system on (I think you can replace the XN with 00). You may find big lumps of sysex at the start of the file, which can be fx setup or even custom drum tunings, if you aren't sure this stuff is Yamaha specific I'd delete it.
John Allcock