Sure you can! Just work from a safe copy of your original.
Often advisable, unless there's no need to keep the previous size.
Most text flows inside text boxes.
Indeed it does, and usually also around images, which may then cause them to be in the wrong places.
So if you change your page size (and if necessary adjust your margins)
There's usually no need to do that. Unless they are wrong to begin with.
your text should reflow automatically.
Indeed. But if one wants a layout where new chapters begin on the right hand side, it's best to check they still do.
Likely you'll have to manually adjust your title page
and rebuild your Table of Contents (things will be on new pages now)
Yes, that's true, and it's best to upload a PDF so they stay that way.
but it shouldn't be too much effort.
Usually it's not. One can just zoom out to see many pages at a time for an initial check.
When publishing on Lulu, it might be easiest to make a new project rather than edit an existing one to a new size.
Not really, but always remember to delete the old file of pages before adding the new on, or the Wizard will try to merge them.
Plus, you can't use an existing ISBN for a new size. That's an ISBN rule, not a Lulu restriction.
You can if the book has not yet been fully Published, or Approved.