Solely increasing the capacity of the truss bottom chord may not provide much, if any, additional ceiling support capacity.
Adding load to the bottom chord of any truss increases loads to the panel point connections and to all other members of the truss.
Most prefab trusses are designed to minimize cost. They use the minimum framing size and gang plate that is required to support the loads. This means that there often is little aditional capacity at the connections or members.
With that said truss mfgs use standard connectors and framing members; they don't fabricate a gang nail plate for each truss design so there may be some additional capacity. For example if a truss design required a 2x3 wood member they would use a 2x4, they don't cut the 2x4 to fit the minimum size requirement. Your truss might be able to carry some additional load without repair. Additionally, with analysis, you might need only repair one or two framing members or panel point connections, per truss.
If you know who mfg your truss you could go back to them with your required new loading and see what happens. If the trusses are not too old the fabricator may have saved your orignal design making it possible to do a very quick analysis.
If your truss CC spacing is 8' remember to add in the load for intermediate framing members required to support your gyp-bd.
If you have steel trusses there are some simple ways to add strength the members and connections in situ. It is not so hard in wood either.
