There’s no way I know to get the same particles go from “Text-to-Sand” (that’s what you should look for as a tutorial BTW) to “Sand-to-Text” with different “Texts” as sources.
In AE you’ll have to transition the particles during the burst out and blend in to the next animation forming in.
It’s usually done with Trapcode Form. It’s the most control you’ll have for that within AE. (its not easily done though)
Cassius Marques
http://www.zapfilmes.com