I know its common practice to use larger LB with reducers and have done that many times myself, but how are you going to support the LB? The conduit is only permitted to support conduit bodies of the same trade size as the conduit. See 352.10(H) for rigid nonmetallic conduit and Exception #1 to 314.23(E) for rigid, IMC and EMT.
Don


Don(resqcapt19)