I've discovered that when switching between Create forms for the Opportunity Product entity, the value in the Currency field is cleared out, and the Existing Product lookup shows all Price List Items in the system instead of just the PLI records tied to the Price List defined at the Opportunity level.  Because of this issue, users that have access to more than one Opportunity Product form aren't able to save the record if they need to switch to a different form.

After creating a support ticket, a Microsoft tech said that this was designed behavior.  It would be nice if this was fixed to allow users to switch between Opportunity Product Create forms without issue.