Why when changing a resource does it change the 'Budgeted at completion' to equal the duration?

Member for

13 years 9 months

Try go to the Projects Details, Resources tab, and tick the Resources can edit activity assignment percent complete..

 

Hope this can help.

Member for

15 years 11 months

Dave,

Make the Max Units/Time of the resource the same as the previous. 

Hope this will help.