Is it possible to change the frequency?

Hmm i suppose one could offer that. Simply a matter of a dropdown and then unscheduling and rescheduling. HOWEVER:

• It wouldn’t be very permanent. WP or The plugin that creates the job may well reschedule it every now and then, say on every update or activation, if not more often (wp does).
• Might be a risk if someone chose say a custom schedule that another plugin had added and then that plugin gets deactivated, the job may well not run?
• Most jobs don’t create too much of a load (with the exception of one of mine ;), soon to be revised ) and if they do, hopefully the creating plugin should offer options as to frequency (as mine does)

Bearing the above in mind, I don’t think I should add the functionality – it would create a false sense of control.

I think of this more as a development tool – helps force jobs to run while one is testing, see whether one’s function is there etc.