First, the pattern is slashed and spread and a dart is added to the non-darted tee, using the method in Fit For Real People (Palmer/Pletsch), shown here. The alteration shown is a large FBA, even for me, but the pattern I started with was about 1-1/2 sizes down from my usual size and required a bit more extra width.
In order to transfer the horizontal bust dart to the center front shirring, the pattern is slashed 3 times at the shirred center front. The slashes are cut up to, but not through, the bust point (where the horizontal bust dart is currently pointing to). Three slashes are added so the dart can be transferred incrementally over the bust area instead of transferring the dart to just one place.
The horizontal bust dart is closed by sliding the dart "edges" toward the center of the dart. As this dart is closed, the slashes added in the step above spread open, transferring the dart to the shirred front area and at the same time, adding additional length to the area which will be shirred. The additional length is needed by full busts, so this is a good thing!
Spread the slashes evenly (they are not even in this pic!), add additional pattern material underneath and secure the alterations to save as your final pattern.
This is the finished top. The full bust alteration adds needed width at the bust and also extra length to go over a larger bust. By transferring the horizontal dart to the center front shirring, the style of the original pattern is unchanged.