Our department is being relocated across the country so we are all losing our jobs. We have been offered retention pay to stay through the transition. We received letters with dates, weeks of retention and severance information signed by HR a few months ago.
The transition has been delayed a couple months. We received letters with new dates with a couple more weeks of retention pay. We were not asked if we were ok with new letters or dates. We were promised in the beginning that nothing would effect our retention payout if the transition were extended. We asked for signed contracts from HR so we were comfortable that nothing could change and we were told that they would not provide that. The letters we received were enough because they had HR signature.
We now have two letters with different end dates and amounts.
Questions:
Does the second letter automatically void the first letter, even though we did do not agree. The only difference in the letters other than the dates and amount is that it says "revised" in the corner. But it doesn't refer to what is beng revised.
We now possess two letters, with different dates and amounts. Are both contracts and should we expect to receive full payment from both letters?
Thanks for any information you can provide.

The only way to be certain that these letters rise to the level of a contract is to take the letters to a local attorney to be read. Contract law is very specific to the laws of the state involved, and to the exact wording of all supporting documents.

I can say the same general thing happened at a company I was working at in the 1990s. At least two people I know of took the company to court to using the same argument you just used and lost. Of course, this was a different state, almost certainly a different wording to the documents and certainly a different judge (should this go to court). This is not the sort of thing where there is a nice clean one-size-fits-all-answer.