When I create a project, I make sure it has a unique marker somewhere, like a unique tag, status, context, whatever. That way, I can go back and isolate that project and multi-edit it. For instance, I can give each one a unique tag.
If you aren't using Stars for anything else, this is probably the fastest way to id them and multi-edit. Star them all, then multi-edit, and destar if you want.
If you are using stars, and have no other unique field, you can multi-edit all your starred task and give them a unique marker of some kind, then de-star them, then star your project tasks, multi-edit, give them a unique identifier, then destar them, then bring back up the old starred tasks and star them. Kind of a tedious workaround, but probably faster than individually editing every task in your project if there are a lot of them.
If you need more detailed instructions, just let me know.