Did you know them persoanlly or were they law office personnel called in to sign? When we’ve had ours done, the lawyer just had to staff people come in and witness our signatures. We have never had friends or relatives there to witness anything. If you did not know them personally, as in our case, I am not sure what they could add if called as witnesses. If your will is still valid (no new spouse, etc.) I would think it would stand on it’s own merit but your mileage may vary. I guess it could depend on if you have some relatives who are questioning your parenting, threatening to call child services, etc. If that is not the case, I don’t know why you’d need to change it unless I am missing something.
And to get back to your original question, I have no experience with those do it yourself will companies. If you decide to use them, make sure they are valid in your state.