What Causes Prolonged Delay In Periods While On Cerazette?

I have been on cerazette for the past two years with no periods or breakthrough bleeding. I want to change to a more long acting hormonal contraceptive and I am hoping to have a mirena iud inserted. I was just wondering when I should attend the family planning clinic for insertion as I do not bleed at all? I also have no children and was wondering will insertion be difficult as my cervix would be closed with no regular periods/previous deliveries. Also do I need to continue taking cerazette pills right up until the day of insertion?