This is one of those random procedures that, hopefully, you only have to do once. The scenario is that you rebottled some magnums into 750s.
The trick is to use a Journal Entry check similar to what you use when you record a bottle run. Enter what was unbottled as a negative number, and then the final amount you made in this process. It is likely that you had some spillage and sipage, and it is likely that there will be a cost difference. Record this different on the expenses tab, using the Inventory Adj account. Enter either a positive or negative number so that you have a zero dollar check.
Here is a video to show you the steps