I've been seeing a lot of wrong statements regarding Fire Emblem: Shadow Dragon's added combat systems with the recent announcement of the new FE 3 DS remake. First, a game is a test of skill with game mechanics that exist independent of plot and characters. There are two ways to analyze a game that has plot/characters. You can rate it objectively as in game mechanics (how balanced or strategic it is), or you can rate it subjectively as in plot/characters. Just calling the whole system 'bad' because of a subjective opinion about not wanting to kill off your units isn't a valid argument. Both aspects of the game system should be considered when forming an opinion. Now whether you like the systems or not, it is a fact that they add strategic depth and difficulty compared to most other FE entries.
The reclassing system adds strategic depth in the form of figuring out optimal classes for each character and setting up your party with classes that compliment each other The only balance issue are the two units Sedgar and Wolf who have unusually high growth rates that make them overpowered if reclassed, but that is a problem with those two units growth rates, not the class change system.
The optional chapter requirements adds strategic depth compared to most Fire Emblems. In this system you need to keep a maximum army size of 15 units to access the optional gaiden chapters. The system adds strategy in the form of choosing which characters you want to live and allowing you to use some units as cannon fodder or choose whether to recruit them or not. Most Fire Emblem games make no distinction whether you lose units or not, except for Fire Emblem 6 and 7, where you are graded on a Survival rank. All Fire Emblem games are easier if you keep the best characters alive, it's no different with SD, you're just more encouraged to kill off the worse or inferior ones, which makes it a strategic choice about what units to keep. The game handing out replacement units is just a way of preventing very poorly skilled players from being unable to progress in the easier difficulty modes - you will still be in trouble if you start losing the best units in hard mode, just like any other Fire Emblem. Thus claims like "the system rewards failure" or "it's less strategic than most other FEs" are objectively false.
As for the games difficulty, it is one of the most difficult FEs on Hard 5. The only FEs more difficult are FE5 SSS rank, FE6 HHM S rank, and FE9 Mania mode. The map design is average for a FE game and provides for a decent amount of strategy. FEDS is a mostly balanced and strategic SRPG. Yes there are some cheap things you can do to reduce the difficulty, but like most FE titles it's not a legitimate challenge anyway since you have limitless turns to grind.
So if you happen to see these things returning in this new FE3 remake, remember that they are not unstrategic or poorly designed. You might hate them because you don't like killing your units off or you think a characters identity should be tied to their class, but that's a matter of opinion.