How to make an old post appear in the feed when its content has been modified?

Hello all

I make significant modifications of old posts and also long reviews to some products that have been already published. I would like to know if is there any way of making them appear in the feed once they have already been published.

Per instance, I have an external xls with hundreds of products with their specification so I create hundreds of custom posts so readers of my site can access that information. What I want is that once I review them they appear in the feed so suscribers can read the review.

I have been searching for it in internet and found that each post has a unique GUID and this is what says wordpress which content has already appear in the feed. But in codex you also say that GUID cannot be modified.

Any suggestion?

Thanks a lot