The SP or "Speed" command sets the slew speed of any or all axis for independent motion (for instance, PR "Position Relative" and PA "Position Absolute" commands).
Say you are in the middle of a PR, or "Position Relative" move and the current slew speed is set to 6000 counts/second, but you want to increase it to 8000 counts/second. One would simply re-issue the SP during the move and the controller will automatically change the motion profiler to accommodate for the new slew speed.
To read up on the SP command and others mentioned here, check out the command reference for the DMC-18x6 (where x stands for the number of axis): http://www.galilmc.com/support/manuals/com18x6/index.html
If you would like to post a short snippet of the code you are using, or let us know what kind of position moves you are issuing, I'd be more than happy to review them to help diagnose why your motion profiler stops when you attempt to change velocity.