If you can re-order the accounts you will not lose the historical data. However, it has been a long time since I have used either the Pro or Premium editions so I am uncertain if you can renumber the accounts. If you have a problem with this you will have to find a Sage SAN member who is using the Accountants Edition to make the changes for you. 

No, in the Pro version I can't change the number of an account that's been used. I'm just moving to the Premium version, but don't have it installed yet, so I'm not sure if it can do this, but I'm suspecting it won't. What is "SAN"? Is there a demo version of the Accountants Edition I could get to do this myself?

SAN is the Sage Accountants Network which includes Accountants and Bookkeepers who are in business and service multiple clients using the various Sage products. The Accountant's Edition does not have a demo version as it is designed for and used by the SAN members when assisting their client base. Sage use to maintain a listing of the Sage Accountants' Network members but I have not seen it for a number of years. If you wish to contact me through the email in my profile we could discuss what it is that you require and a quote for the work would be provided.

SAN is the Sage Accountants Network which includes Accountants and Bookkeepers who are in business and service multiple clients using the various Sage products. The Accountant's Edition does not have a demo version as it is designed for and used by the SAN members when assisting their client base. Sage use to maintain a listing of the Sage Accountants' Network members but I have not seen it for a number of years. If you wish to contact me through the email in my profile we could discuss what it is that you require and a quote for the work would be provided.