I try not to use units that already are an advanced class, but you could use a second seal and change to a base class, level him and THEN promote him to an advanced class, this time with better stats. I ignored Frederick in my first playthrough, but I might honestly do this in my second (hard mode) run. :D Plus, I want different pairings.
Also, the kids can turn out really awesome. Lon'qu and my tactician had a myrmidon who turned out just as good as his father, and inherited mom's magic and res. I made father and son swordmasters because you can never have too many...