When global changing resources - its halves my 'budget at completion' help!

Morning,

I am trying to change the resource in my plan to a different one.  ie I have two resources define, Electrical 1 & Electrical 2.

All the activities with electrical 1 assigned i want to change that resource to electrical 2.

I created a refelction and tested it out and it when it changes the resource it also halves the Budget at Completion.

Any help greatly appreciated.

Dave