You may have to do it in multiple steps to get the results. The sales history may not work the way you want though. Try to export 1 or 2 items with the inventory export using excel, make your changes, as for the old part #'s use "alternate" column in excel, import them after your changes. If the old items can be rendered inactive then do so to keep any history associated with them. You mat have to change some of the quantities to zero first.

We ended up buying MK Powertools to do this.  It's an extremely expensive third party add on but it allows you to change or merge part numbers while retaining all the history.