Hi and welcome to MM
Firstly - it's highly unlikely you have cancer - if you had bleeding because the womb lining had thickened then there might be a small chance that cancer could be developing.
For contraceptive purposes, I believe the Mirena should be changed every 5 years but for HRT purposes it should be changed every 4 years because the progesterone runs out by the 5th year. Before the menopause you would be still be producing some progesterone so this would stop the womb lining from building up, even if the Mirena had run out of progesterone. The Mirena with separate oestrogen as pills, patch of gel is a very good combination. I would have your scan adn have another fitted and then carry on using the oestrogen separately. The Mirena protects the womb lining and is delivering the progesterone more locally to where ti is needed.
Your GP is wrong, you should have had teh Mirena changed more often - he/she has only just realised this so is panicking in case you womb lining has built up!!!! These GPs really should get up to speed on treating the menopause and the Mirena has always had a maximum life of 5 years anyway - where did he get the 10 year time scale!!!!?
Here is the info from this site about teh Mirena as part fo HRT:
MIRENA
Mirena is a levonorgestrel (type of progestogen) releasing system which sits inside the womb, gradually releasing the progestogen into the womb. It is licensed in the UK and Ireland as a contraceptive agent, for treatment of heavy periods and, from August 2004, also for the progestogen component of HRT. It can be used in both the perimenopause and postmenopause and it is particularly useful for:
Persistent progestogenic side effects from systemic HRT despite changes in type and route of progestogen.
When contraception is required along with HRT in the perimenopause.
When withdrawal bleeds on sequential HRT are heavy, after investigation if indicated. (see WHEN TO BE REFERRED )
With Mirena in place, systemic estrogen alone can be taken as the Mirena provides adequate protection of the womb lining and the estrogen dose and route can be tailored to meet the individual's needs.
Progestogenic absorption throughout the body is minimal so reducing progestogenic side effects. The effect of Mirena on the womb lining can significantly reduce bleeding and when used as part of an HRT regimen, in time, 30 to 60% of women have no bleeding at all. Although Mirena used for contraception is licensed for 5 years, the license for use for the progestogen part of HRT is currently 4 years.
Price: £88.00
If you get on well with the Mirena with separate oestrogen, I would simply have a new Mirena fitted - this is such a great non hassle HRT regime. DG x