I get this question often. As you might guess, the answer differs based on the association. Overall, you want to convert data that is well vetted and trusted.
If you have any policies that require the validation of historical data, you will need to convert the amount of history to support the policy. For example, if you have a policy that someone must have served on a committee in the past 5 years before being nominated to the board, you will need to convert 5 years of committee history. But understand: that doesn’t mean you need to convert 5 years of all history.
For data that does not have a policy associated or a reporting requirement, we recommend converting no more than 2-3 years of activity-based data (e.g., committee involvement, registration history, purchases).
As for demographic data (e.g., gender, topics of interests, career categories), you should only convert history that has been updated in the past 2 years. Some associations opt not to convert demographic data. Instead, they invite the constituents to visit their personal profiles and update their own information at go live. By doing this, you get the most current information and introduce them to the new interface.
Flickr photo by Gilad Lotan