There is an OpenEdge product called DataServer that allows an ABL client to connect to an Oracle or SQL Server back end. I have never bought it or used it so I can't advise you further. But my understanding from hearing others describe it is that implementing it is not trivial or transparent to your application and you may have to write back-end-specific code to get queries to perform acceptably.
You should research it and conduct a cost-benefit analysis, including first-hand information on costs and potential pitfalls, before committing to changing your back end.