Depends on which modules you want to learn. I have transitioned as a JDE Financials Consultant to EBS in the last 3 years. Only working on R12.
Get some education:
There are some CBT courses available from Oracle University as well as classroom based courses.
Regardless of the Apps modules you want to learn, I recommend doing the System Administrators course and SQL fundamentals. If you have a knowledge or Oracle database 9i, 10g etc.. and Unix/Linux that would also be beneficial.
How much will your JDE knowledge be useful ?
Be prepared for a shock. Compared to JDE, Oracle EBS Financials has many more features and functions, along with many integrations with other modules that you will find completely different to JDE. Similarly with Projects, HR and Payroll. Supply chain modules seem to be similar in feature/function and process to JDE.
I found it difficult to learn by reading the manuals. Which now come either as an implementation guide or User guide. You have to read both to get the full picture. As with JDE there are many ways of setting up the modules to achieve a Business process, which you will need to learn if you want to implement the software. You will also learn that you need a good Apps DBA to do all the technical stuff, very similar to CNC in JDE. I have some CNC skills and can do development in JDE but have not even attempted to learn these area’s in Oracle EBS.
Oracle EBS has many modules and to learn them all will take many years. Best to pick an area and specialize in that area. There are some Oracle EBS generalists out there but they will all have 10 years plus experience. Best way to learn is hands on.