Are there any stipulations for changing the sacred thread (Yagnopavitam)? For example, must it be worn before partaking food or before Madhyanikam?

Usually, the Yagnopavitam is changed in the early part of the day or forenoon; it is a practice to continue with daily prescribed duties after changing Yagnopavitam. It is better to perform Madhyanikam after changing the Yagnopavitam.