To answer the second part of your question, once beta ends, it is expected that we will have an established tag system. The users who have been around during the beta will know more or less what everything should be called, and the new users will have the autocomplete to guide them. At that point, retagging should be restricted to bring new, poorly-labeled questions in line with what everyone else expects.
At this point, retagging is likely to happen. If there is only one question tagged
linear-least-squares and one labeled
ordinary-least-squares, autocomplete won't be much help to someone trying to figure out how to tag their new question. At the same time, the total number of questions is low enough that we don't need to rely on tags as much in order to avoid being overwhelmed. I wouldn't worry too much about it just yet.
I think we will eventually need a different system from MathOverflow, but if that's what many people here are used to, it could be a good place to start.