When the growing tips of Marginata stems discolor or die back, it is a sure sign of damaged roots. Most commonly, that is a result of keeping the soil a bit too damp for an extended period of time. This is a common phenomenon with older, established Marginatas.
I don't mean to be critical but replacing the soil was the worst thing you could possibly do. The soil was not the problem. In removing the soil, you no doubt inadvertently damaged many of the tiny root hairs that do most of the work. I don't have the details, but the chances are the pot is too large and the potting mix is not porous enough. I hate to write this, but I expect that your Marginata will now go into a more serious decline. I hope I am wrong, but be warned.
