Rerouting with another carrier is your only chance since you have self-connections both at TUN as KUL. When I was rerouted with Turkish last year, not any problem to get original route crediting with FB - my new ticket clearly stated 'invol. rerouting' or so. In the thread I showed above, there was rerouting with TK, QR, EK. Always prep yourself - that works quicker with agents that may be under stress. They will tell if some option you suggest is not available. I would look for some connections that suit you with these carriers, call, hope you get an agent that understands and helps, and then indicate what you want. I opted for TK - was an A320 2-2 recliner in J to Istanbul but the long-haul to BKK was really a long lie flat flight -with Gulf carriers you have two 6-7 hour flights (both lie-flat)