Your experience may vary. It will depend on SAS features your site has implemented. And the results of a migration are not always just related to code change - consider some subtle SAS ODS differences you may experience along the way.
For example, a SAS v6 (from) migration to v8 was not always as straightforward as v8 to v9.
Suggest you search the archives using keyword "migration".
That's why I said "mostly straightforward".
If we are talking about code, you shouldn't much trouble unless you have some particular and complex implementations. I mean, v6 datastep coding is pretty much the same as v9 and the majority of procedures also.
If we're talking about SAS server architecture, then forget it. v9 brought a complete new setup, you'll probably have a lot of manual work if not to redo many things from scratch.