The reclass system in FE11/12 might seem overwhelming, but if you take the time to learn how it works and use it to your advantage, you will not regret it.
Typical reclassing scenarios:
Building up different weapon ranks
Adjusting a unit’s growth rates
Balancing stats that are too high or too low
Dealing with unusual terrain
Exploiting enemies’ weaknesses or the weapon triangle
Getting around maximum stats
Planning for the future:
Secondary weapon ranks from promotion can be difficult to train up, but you can get a head start through reclassing.
Growth rates are affected by reclassing. Everyone has their own base growth rates, but their current class modifies them. A summary of the most important ones:
All magic classes (+10% to +30%)
NOTE: all non-magic classes lower MAG growth
It’s probably not worth the trouble of reclassing non-magic users to get their MAG up for levin swords.
LCK: there is no change to LCK growth when reclassing.
Arms scrolls maximization:
If you want to use an arms scroll to boost a Falcoknight’s sword rank from E to D, reclass her to swordmaster before using it. Swordmasters have a base sword rank of C, so she’ll get 60 WEXP going from C to B whereas she’d only gain 30 WEXP using it to go from E to D. Needless to say, I wouldn’t mention this trick if the extra 30 WEXP didn’t carry over. This trick works best for bows and swords because Swordmasters and Snipers have a base weapon rank of C in their respective weapons.
Walk on water:
Pirates and Berserkers are by far the best foot units for this, but in a pinch, Snipers, Heroes, and Swordmasters are willing to get their feet wet in rivers, but not seas. Of course, fliers don’t care about water, but sometimes the presence of effective weapons will limit where they can safely go without Iote’s Shield.
(Serenesforest.net was used as reference and to double check things because my memory alone isn’t that good.)
Everything you would ever need to know about freeclassing, courtesy of moldyonion. Be sure to thank him when this saves your hide in higher difficulties.