I don’t suggest you to take this drastic step if you are happy and satisfied with your current profile. There is a dearth of mainframe professionals around the globe and the demand still exists. Are you a technical consultant in Mainframes? If yes, stick to it.
If you move to SAP ABAP, you will be starting your career from scratch, and what are you planning in ABAP, functional or technical? Although scope is higher here as compared to Mainframes but so is the availability of professionals.
Think over it!