Now every new record will get its own serial number. If you've already got records in your DB, don't worry. It's pretty easy to redo them. This is a precaution, because Replace is not undoable. Once done it can't be reversed, so take your time with those choices.
Serial numbers can be either text or numbers.
For now, though, either will do. Text has one disadvantage though, in that a Find on a text field will find all matches that begin with the characters, and ignoring zeros after numbers. So, it will find both 20 and 2. This is easily fixed by beginning the text serial numbers with a few zeros. You can even put a letter to identify which field it is, for example Contact ID's could be C0001.