There shouldn't be any problems what so ever. When shooting in different formats, the greater concern is actually frame rate. Since you have both shot this at 24p, you won't need to convert frame rates.
The first step you should take would be to inform your editor of the error, and clearly indicate on the DVCpro tapes which are 1080i and which are 720p. Most editing software can preform a digital conversion while ingesting, allowing to "down convert" the footage to 720p. The editor may have to capture the 1080i footage in a separate project, and then export into the correct format, then import into the main project using the correct format. If the editing is being done at a professional suite, they may have available an up-down-cross converter, such as the Leitch X75, that can be used to convert the footage deck-to-deck, or while ingesting.
I hope you find this helpful,
As you might imagine - this is very good news to hear. One additional question: will there be ANY perceptible difference to this footage in its appearance as compared to the 720P/24p footage already shot?
I would imagine that there would be no discernible difference in the video quality. Where you might run into quality discrepancies is if you were upconverting the 720p footage to 1080i. Since you are downconverting you really shouldn't notice a thing