The data you are entering must be accurate as of the conversion date you select. Ideally, this date would be the beginning of your fiscal year. Or, a little less ideally, this date would be the beginning of a fiscal period.
One special feature of Sage 50 is that you can have two years open at once (up to 26 periods). This means you can keep last year's records accessible. It also gives you a lot of flexibility as to when you convert. For example, you can set the first accounting period to the first period of your fiscal year. This will make next year's closing easier to manage.