There are two occasions for mixing a 62.5/125µm multimode fiber and a 50/125µm multimode fiber. One is that the light needs to go into the 62.5/125µm fiber from the 50/125µm fiber, and the other is that the light goes into the 50/125µm fiber from 62.5/125µm fiber.
For the first case, the smaller core of the 50/125µm fiber can easily couple to the 62.5/125µm fiber and is very insensitive to offset and angular misalignment. Therefore, nearly no problem will happen. However, one big issue, link failing, may occur when upgrading to a 50/125µm multimode fiber from a 62.5/125µm one. When a 62.5/125µm multimode fiber is being mixed with a 50/125µm multimode fiber, the light in the 62.5/125µm fiber will escape into the cladding of the 50/125µm multimode fiber, which can cause a coupling loss. If the loss is relatively huge, it is not acceptable in mixing the 62.5μm and 50μm fibers.
Then, how to tell the feasibility of the mixed link with a lower coupling loss? Actually, the traditional range of mismatch coupling losses has been covered in several documents such as the Fiber Optic Technicians Manual published by Delmar in August, 2005, stipulating the range from 0.9 dB to 1.6 dB when mixing the 62.5/125µm multimode fiber and 50/125µm multimode fiber. If the actual loss exceeds the range, it is not recommended to mix 62.5μm fibers to 50μm fibers.