You may need to adjust maxwalkspeed on the fly. I needed to in order to create sprint and slow walk functions. If that is kinda of what you are after.
float MaxWalkSpeed The maximum ground speed when walking. Also determines maximum lateral speed when falling.
As @BadScientist89 mentions below. i adjust maxwalkspeed - maxacceleration - breakingfriction - groundfriction before applying my movement and adjusting my globalanimratescale.