The answer depends on the structural capacity of the roofs supporting framework. An onsite inspection by a qualified engineer should be conducted. If the supporting joists are 2x6 and have no damage then more than likely one could be reasonably sure that there would be no problems.
It is not preferable to install metal roofing over composite shingles, but it can be done if strapping or sleepers are first fastened on top of the existing shingles into the roof deck rafters. The strapping will create a level surface to attach the metal roofing.
How many layers of shingles. If there is only one layer then I would say yes. (We've had this done on our house and they warranted it for 25 years.) If you have more than one layer then ask an expert for advice.