Impact on Historical Reporting when re-purposing the Type field on the Opportunity Object

I have a client that used the Opportuninty Type field more as a category definition than a set of values that would drive page layouts when using dynamic layouts.  We now want to take advantage of dynamic layouts but do not want to lose the existing data.  I'm considering the idea of using an indexed picklist for the category data (configuring the picklist, exporting the data from the Type field and importing it into the new indexed field), and then changing the values in the Type field for the dynamic layout values.  However, I can't remember the impact this has