Hello UKcheetah,
For a study on muscle plasticity showing a fiber type shift 2b -> 2a (it seems that researchers are replacing the notation 2x for 2b), see: Maffiuletti et al., Neuromuscular Adaptations to Electrostimulation Resistance Training, 2006 American Journal of Physical Medicine & Rehabilitation; the study employed EMS at 75 Hz.
For a study showing the opposite change, fiber type 2a -> 1, see Nuhr et al., Functional and biochemical properties of chronically stimulated human skeletal muscle, 2003 European Journal of Applied Physiology; the study employed 15 Hz EMS.
The first study was conducted over 4-8 weeks, and its author suggested that previous studies limiting EMS to 4 weeks were not able to produce significant results.
Best regards,
Giovanni Ciriani