The CITI Hilton cards are to change to Amex cards. (This mentioned in the Any benefit to Additional Amex cards thread, but the issues are broader and that title will not reach many).
Here are my thoughts.
If you have a fee card you may wish to consider whether you want it (answer is probably yes).
A major cost to this conversion for some is that I understand Amex does not give new credit cards (charge cards are exempted) once you have 4 cards. I doubt if this rule will prevent the conversion even if you already have 4 cards.
However, if there was a Amex card I wanted I might want to get it now (I have 3 and once the Hilton is there, I would be blocked from more).
If you do not have a Hilton Amex now you may wish to apply and pick up their current offer (50,000 points for spending $1,000 for their standard card). While this may block the conversion, many would be ahead. If you once had the Amex Hilton you are applying for, you cannot get another bonus.
Their letter says a few will not get a conversion offer (reasons unstated, possibly credit score related, or that you already have a Amex Hilton), or if you are unlikely to accept the new card for some reason, you may ask for a pr0duct conversion to another CII card, or wish to do a little spending on the CITI to get a few points to extend the life of your points.
I believe CITI policy is not to permit transfers of credit lines (except when getting a new card), but some may wish to call and try if they think this would be useful. There may even be a few who apply for a new CITI card now with the idea of offering to transfer the Hilton line to the new card (say a Double Cash card with a 0% offer on new purchases).