DSXchange

We are looking to upgrade from Datastage 6x to Datastage 11.3. Will a direct export-import kind of an option work here or will I have to rewrite code afresh in 11.3 ?

Nominally an export and re-import into a clean installation followed by a complete recompile should be sufficient, but your are bridging a lot of years and there might be the odd hiccup. A lot depends upon whether you have used just a basic set of stages and types of jobs or if you have a lot of exotic code, i.e. BASIC routines or the like which access internal structures. Some of those might have changed over time.

Generally DataStage has done quite well with backward compatibility.

Agreed. However, default behaviours can change over time and loopholes get closed so sometimes people see things that 'ran fine' under older versions no longer do so under newer ones. That because DataStage has tighter checks or code fixes that make old jobs that really shouldn't have been working in the past stop working. Much more of that in the Parallel world than in Server, however.

Still, full regression testing required.