Populate Field on Form using Lookup to Another Entity

I have an entity (E1) with a lookup field and a number field.  The lookup refers to a second entity (E2) which also has a number field.  When a user chooses a value for the lookup field on the E1 form I'd like the number field on that same form to be populated with the number value from the E2 entity using JavaScript (onChange).  I'm fairly certain this will require SOAP (which I've never used).  Thanks!