Can I start by uploading a text-only version of an eBook—to KDP, most likely—and later update the book to include the illustrations? This would let me get the story “out there” more quickly. I should only need one more revision pass to get the text ready.
Yes, you can do this, but do you really want to? One great thing about eBooks is you can upload and publish new versions anytime, with some limitations.
In your case, you might decide to make the two books discrete products—a print version and an illustrated version. But take care to make each version a quality product, putting your best foot forward with what you’ve got to offer. If the book is really lacking without the illustrations, wait until you’re ready to include them.
But yes, it is technically possible to upload new and improved versions of your book. Doing this and nothing else will change the deliverable that readers download. Do you really want to deal with readers’ disappointments over needing to “upgrade” their books? Or worse, pay for an upgrade if you sell the illustrated version separately?
Another consideration is that you want to avoid disappointing the buyers of the non-illustrated version once the illustrated version comes out. You might offer the second version free or at a steep discount. It’s all about building and taking care of your readers and your brand.
So, this is a win-lose situation most of the time.