Since synchronous belts are a positive drive and do not slip, they will attempt to transmit all the load that is applied. V-belts will slip at peak loads, almost acting as a clutch and as such not passing that peak loading on to the system's structure.
Since synchronous belts will not act as a clutch, it is important to make sure that the drive structure is adequate. This is more of a concern for HVAC type applications, given their structural considerations.
To check for structural rigidy, first make sure that the system is powered off, locked down and tagged out. Never touch or work with any belt system without taking appropriate safety measures.
Then, grab the opposing belt spans and push or squeeze them towards each other. While doing this, keep an eye on the shafts in the system. If you see significant movement or deflection, the system is not an ideal candidate for synchronous belt conversion without further structural strengthening. If you do not see any movement, or minimal movement, the system's structure is adequate and a good candidate for converting to synchronous belts.