Hi Andy,
This may seem a bit crude to you, but it works for me. Take the Panel Memory you want to retard from and put it into the next PM position, turn your tempo down and reset the PM. Use your foot switch to make the PM move. If I want a slower retard, perhaps over 4 to 8 measures, I might slow down gradually by taking three steps to do it. Of course you have to be in the Expand mode to carry the tempo info.
Frank