The text of the Landing Page is the only place in a ThankView that will update after a campaign is sent. If you are only updating the Landing page, then a resend is not necessary.
One resend per recipient is possible. If the recipient has not opened the ThankView, you can go back into the campaign and edit what you need to. Press Resend for each recipient that you want to send the ThankView again to in the Sends page of the campaign.
If you keep the Email Subject line in the campaign the same, the emails should combine into one with the newest send being the first one the recipient sees.
We always recommend that our partners send themselves a Test email to see how the ThankView would appear to the recipient before mass sending to prevent the need to resend.