I dunno if people are aware of this or suffer the same problems as I do.
When I made a system transfer to a new 3DS about a week ago, I noticed most of my calling cards and several of my friends were greyed out and thus couldn't visit their castle any longer, even though more than 24 hours have passed. As many of you may or may not be aware it's has to do with the 3DS batteries getting changed and affecting the internal 3DS clock or something. There seems to be very few to none fixes to this, to qoute Masterthunderblade that helped me:
Sadly these didn't work for me as I expected. However, I possible found a third solution on how you can revisit them. It's through the StreetPass Results. Since the streetpass even register when you visit other castles under your Avatars name you'll be able to fool the system with that. But for this to work you need to have fought with the other castle such as seized or won the castle battle them or it won't appear on your result list. I think it also record if you lose a battle but I haven't check that condition yet.
So if you have a friend greyed out, even after 24 hours but you have seized their castle before. Go then to your inbox and check your StreetPass result and check for your Avatar name, with the details "Castle seized!" or "Attackers won!". The text should be black meaning you can visit yourself but it's actually your friends castle. The biggest pain is that you can't see whom you seized.
I just wanted to share this with your folks since I haven't found any other solution other than Masterthunderblade mentioned and let me and others know if it works for you too.