If they're in breach of contract, solve the problem at that level. Reverse enginerring from code can open you up to DCMA problems.

If you're in doubt, get a legal opinion from your contracts people (or your corporate attorneys). Explain what you propose to do, and make sure they understand it involves reverse engineering. They might say "no problem", or they might say "NO WAY." If the latter, better to know that know, before you invest time in hacking Java bytecodes.