As far as I understand if you are put onto HRT before you are 54 and before you have been one year without periods then at age 54 you can be changed to conti HRT because most women will have more or less reached menopause by this age.
This is what Dr Currie says on this site:
"Period free" or continuous combined therapy can be used by women who are 54 + yrs, or more than 1 yr post menopausal at any age. http://www.menopausematters.co.uk/postmeno.phpIf you are under 54 then according to this you would need to wait a year to see if you had another period - but if you don't want to do this, then a cyclical preparation is usually advised.
In some cases - like honeybun - she was put onto a conti HRT before age 54.This can lead to more irregular bleeding than expected - but only if you are not yet post-menopausal.
It sounds like your doc is happy for you to try a conti HRT though from what you say?
The other alternative is just to have another Mirena put in, if this suited you and then take oestrogen only HRT eg Elleste or whatever type you prefer.
Hurdity x