If one misses the Yearly Upakarma (sacred thread changing ceremony) rituals on that given day, is there any alternative day one can perform that ? After one week or month ?
If someone misses the upAkarma, he can perform in the next Poornima / full moon.
I am adding a message from one Sri Vaishnava website, though it doesn't have scripture reference but this custom is followed by all.
If for any reason you have missed performing the upAkarma on Avani-Avittam (for eg. Grahanam etc.,), you can perform it on the next pournami day. Upakarma is best performed in a river (rather than in a temple). This year at Buffalo, 20 of us joined together and performed the upAkarma on the Niagara river at the Beaver Island State Park. It was a great experience and everyone of us enjoyed it. For a moment I thought the river must have felt thankful for serving the cause of Sanatana Dharma!