I don't think you would have to start from scratch in SAP - especially if you target an area of SAP that fits in well with your core skills. Given your strengths in Java and Retail, you want to focus on getting SAP NetWeaver exposure using the Java-enabled aspects of the product line. You might find some opportunities working with NetWeaver Portals, or Web Dynpro for ABAP.
The latest version of NetWeaver, version 7.1, features the NetWeaver CE (Composition Environment), which is the first software product based on Java EE 5 (besides Sun's own). This might give you a valuable entry point into SAP if you can find your way into one of these cutting edge NetWeaver environments. Your retail background might also come in handy. SAP Retail is hot right now, and there is definitely custom development work in Retail that could play to your strengths - though you might need to pick up some ABAP skills as well. It might be a challenge for you to break into the SAP technical side, but you won't have to start from scratch.
This was first published in October 2007