With questions like this it's best to start with the Platform Availability Guide(s), product life cycle info, and the KB.
PAG landing page and product life cycles:
OpenEdge Product Availability Guides and Life Cycle Guide - Wiki - OpenEdge General - Progress Community
KB:
Which versions of Progress are supported against what AIX platforms?
Knowledge Article
http://knowledgebase.progress.com/articles/Article/P20529/p
My opinion: I don't like to do in-place OS upgrades on servers. How do you know it will even work on old hardware and what is your back-out plan if it doesn't? I also don't like to run very old applications on OSes they were never designed or certified to work with. Progress 9.1D is so old (retired over 11 years ago) that it wasn't even certified with AIX 5.3, much less 7.1. If it works, it's by chance. You're likely to run into issues with Progress taking dependencies on old OS shared libraries, old java version, etc., which you won't have in the new OS.
If I were you I would first focus on the significant business risk of running a business application on a platform that has been retired for over a decade. Work with QAD to upgrade to OpenEdge 11.6 as part of a server migration project; old servers run 9.1D/5.3; new ones run 11.6/7.1. Then the back-out plan is easy: keep running production where it is.
That approach might also entail an application upgrade; I don't know. You would have to work with QAD on that. Business applications should run on supported application platforms and operating systems. It is tempting fate to do otherwise.
