<b>What all should I consider while going for migration?</b>
First and foremost, your considerations should be driven by your reasons for migrating. If it is as simple as getting off of Synon, then I would suggest you look for a web architecture that brings your applications up to speed with the technologies of the day, and offers a growth path as that technology continues to evolve.
I would also encourage you to think about how you want this application to behave in today's world. Chances are that if the application was written with Synon, a lot has changed since the time it was written. Web architectures are by nature event driven while most Synon designed apps are process driven. You need to think through how conversations should change to account for this. While doing so, you should also consider not just your application "as is" situation, but the application "should be" situation. In other words, if I were writing the application from scratch today, how would I want it to work. Depending on the approach you take, getting to the "should be" situation may not be as difficult as one might think.
We have some experience migrating DB2/400 Synon 2E applications to a java web architecture complete with separate presentations for PC's, tablets, and smartphones.
<b>Do we have any tools which help me in this migration?</b>
In my case, since I am not a java programmer, I used a tool called m-Power, written by mrc in Lombard, IL. It speeds the migration process, insulates me having to learn any programming syntax, and allows me to concentrate on building the application from the data-base up. I think about the application conversation, security, flow, etc. and not about specific programming syntax.
<b>Let’s say if I want to migrate the database to Oracle? Do we have any tool which helps me in database migration?</b>
I am not an Oracle expert. If you use triggers, constraints, UDF's, wrappers, special field types like date and timestamps, etc., the job will be larger if want to to migrate to another data-base, than if you just stay on the data-base you know. Most data-bases today let you do a lot of the application data integrity checking in the data-base itself. If you wish to avail yourself of that, you will have some additional learning curve.
Although I have not used it on other data-bases, the m-Power tool I use works across a wide variety of data-bases and does allow you to create and move tables from one db to another. It also knows the difference in data-base syntax for defining things like UDF's wrappers and special field types.
<b>Which is the preferred technology to which can be migrated?</b>
I can only speak from my experience, but I have been very happy using my DB2/400 data-base and web 2.0 java servlets built with m-Power. It's a cost effective solution that can move a business much farther down the migration path to applications using today's technology and ready to take advantage of technology of the future. I also like it because I do not need m-Power to run the applications. They can be deployed anywhere with no licensing fees to mrc. And the code they run, Java, SQL, and html will be around a while so I don't need to worry about technological obsolescense.
<b>How can I migrate DDS (Screens) and to what GUI?</b>
Hope that helps.