If Tru Johnson were to get moved to safety for the 2013 season, it would be a wise move. He is 6'2 and this year was just his rookie year. I think that he could be a good safety for the Rams next year, or at least better than Craig Dahl.
It looks like Jarius Byrd coming to the Rams probably won't happen and Darian Stewart and Craig Dahl's contracts are both up. Stewart is a good special teams guy so you should re-sign him. For the third corner, just re-sign Bradley Fletcher and that will take care of that. Moving Tru to safety would also keep the Rams from taking a safety in the early rounds of the draft.
Anyway, that's my take on the safety situation in St. Louis. Heck, if anything you might as well try it out in the preseason. Thanks for reading!