If you create a new plant then why would you need to change the MRP type?
You are still doing business with the old plant until your go-live, and you need to create new material master plant views for your new plant anyway.
MRP run is at plant level. As long you do not schedule a MRP run for the new plant (this is an activity of the cut-over plan for the go-live date after stock load is completed) you wont get any negative effect .