How to change the scope of the SYSIKJUA enq

Answer

As long as all the affected systems are in the same SYSPLEX, you do not need to reIPL them in order to install RNL changes.
In other words, if the SYSPLEX matches the GRS complex, you can use the SET GRSRNL command to make the change dynamically; no system needs to reIPL.

However, the following corresponding decisions by the system are to be noted:

After the operator issues the SET GRSRNL command, GRS suspends any job that requests any affected resource (a resource that is not the same in the current RNL as in the new ones) until the change completes or until the operator cancels the change. If a job currently holds one or more of the affected resources, the change is delayed until that job frees any affected resources

The z/OS MVS Planning GRS manual explains that to change the RNLs currently being used by global resource serialization, set up the GRSRNLxx parmlib members with the new RNLs.
Next, issue the SET GRSRNL command on a system that has access to those members. The new RNLs are then communicated to all systems in the complex.

Note: Even though only one system needs the updated parmlib members to start the change, be sure to copy the updated GRSRNLxx parmlib members to each system's parmlib. Any system that needs to can then IPL again into the same complex. Otherwise, the change will be in effect only for the duration of the IPL.