If you are brand new to the game, welcome! My first suggestion would be that changing defense is a guaranteed way to ruin a season. If you are at a your destination school at D3, then do whatever makes you happy and do it now! If you are thinking you may move to a different school (excepting a really poor season, you will probably be eligible for all of D2 & D3 next season), then I suggest just run and practice the O&D your team currently plays and figure the game out from there.
If you've started with a bad team, just changing bad players to a more suitable system won't make them good. It will, in fact, make them worse while their defensive IQs improve from horrible to mediocre (e.g. D to B+).