A client has a number of properties which are being sold to new owners, and they wanted to know how to handle that in th software without disturbing existing transaction history against the old owner.

My suggestion is to create a whole new property.  Maybe use the Change Codes menu option from the maintenance menu to change the code of the existing property from X to X-old or something like that.  Then create a new property called X and associate it with the new owner.  On the date of purchase, flag the old property as no longer being managed by you.  Then switch the property code on the tenant details screen (assuming the tenant is staying) to the new property.  Or maybe create a new tenant as well.