Is spasticity curable or reversible, or is it something that can dissipate over time with rehab exercises through neuroplasticity?

Spasticity is the nervous system's response to damage to upper motor neuronal pathways. Once established, it is not structurally modifiable, but will vary in response to various internal and external stimuli. It is often controllable and rehabilitation is a valuable tool. We do have anti-spasticity medications, and cannabinoids are being studied. A baclofen pump may be effective.

Maybe. The answer to your question is dependent upon what is the cause of or the diagnosis related to your spasticity. In general, spasticity can be helped with therapy and home exercises and medication interventions and this should be managed and treated by a provider familiar with this condition.