Change a NEMA 6-15 plug to fit an NEMA 14-50 receptacle
You can't do that.  You can't put at 50A plug on a 15A appliance.  

What you can do is change the NEMA 14-50 to a NEMA 6-15.  You will also need to change the breaker in the panel, and will need to pigtail the wires because #6 wires will not fit on either a 15A or 20A breaker or a NEMA 6-15 recep.

Now, if you need both sockets to be active, you can install a small subpanel at the NEMA 14-50 site.  That subpanel will have 2 breakers: a 2-pole 50A and a 2-pole 15A or 20A.  The 50A will supply the NEMA 14-50 recep.  The 15A/20A will supply the dual NEMA 6-15.