If you compile a bitcode file from anywhere around LLVM 3.3 or later, you can generally read it in the most up-to-date version of LLVM (you may have to first strip debugging metadata though).
The LLVM-C API also maintains compatibility, but that comes at the steep cost of preventing you from using most of the power: LLVM-C is basically limited to reading/writing LLVM code, running passes largely at the -O1/-O2/-O3 level of granularity, and executing the JIT. Writing a pass with that API is very difficult, and pretty much impossible if you want to use any analyses.
The LLVM-C API also maintains compatibility, but that comes at the steep cost of preventing you from using most of the power: LLVM-C is basically limited to reading/writing LLVM code, running passes largely at the -O1/-O2/-O3 level of granularity, and executing the JIT. Writing a pass with that API is very difficult, and pretty much impossible if you want to use any analyses.