Since only physical interfaces (candidates to be assigned to an IRF logical port) operating at the same speed rate can be assigned to the same existing IRF logical port you can't just simply add your new 40Gbps QSFP+ port(s) to a existing group(s) of 10Gbps SFP+ port(s) already bound to the existing IRF logical port(s)...this mean you can't plug QSFP+ and add it to IRF Port 1 and IRF Port 2 if those ports are operating, each one, with one or more SFP+ physical link.
If you have a Chain IRF Topology I think you will be forced to totally (re)configure the actual IRF and this will clearly require a disruption...did you deployed a Chain IRF Topology (IRF Member 1 IRF Port 1 to IRF Member 2 IRF Port 2) or a Ring IRF topology (IRF Member 1 IRF Port 1 to IRF Member 2 IRF Port 2 AND IRF Member 2 IRF Port 1 to IRF Member 1 IRF Port 2 to close the loop)?