I just added the HD Extra Pak yesterday and got the first 3 months for free offer. I too have a grandfathered package. If you try to add it on the web site, you will get a message that you have to change your package to a current one in order to add the HD Extra Pak. I called and the automated system actually stated their current first 3 months free offer. When I got connected to a CSR, she had quite a bit of trouble finding the offer on her system. She kept asking me questions about the offer - was it for new subscribers only, connected with any other offers, etc. I told her what I heard on their automated system right before she picked me up. She finally found it and added it without having to change my grandfathered package. Actually, the subject of changing packages never came up in my discussions with her. Logged on to my Directv account and saw that the package had indeed been added and that the current charge was $0.00.
Definitely call to add it if you don't want to change your grandfathered package.