Hm... Okay, if you create the new products and upload their files while leaving the original products and their files in place as you'd described, then once you have the updated products in place, you should be able to change the Item Numbers of the old products, so you can give their original Item Numbers to the new products which are replacing them, and that would allow you to retain your existing button codes.
E.g., suppose you have some download product with Item Number "ABC", and then you create a separate download product "ABC-new". Then if you change the Item Number for "ABC" to "ABC-old" and change "ABC-new" to "ABC", buyers who purchased the original "ABC" would still get the old file. Once the buyers of the old version have received their files, you can finally delete "ABC-old".
Bear in mind that this means you would need to have a subscription plan level with adequate storage space for both the old and new versions of your product files, at least temporarily.
As another, much simpler alternative, you could simply close your E-junkie store to any new sales temporarily while you're overwriting product files for your existing products. This should not affect pending downloads for completed sales. In your Seller Admin > Account Preferences, you can "Make all items unavailable for purchase until" some date and time you set.
