The difference is whether there is bleeding or not.
I am guessing you had the ultrasound due to bleeding, and now protocol is being followed re the results.
The best thing would have been not to scan unless and until it had gone on for more than 6 months, except if you have other risk factors such as obesity or previous anovulatory disorders eg PCOS, in which case your GP would have been erring on the side of caution.
Being on sequi for 15 years is also classed as a risk factor so this may have had a bearing on the referral too.
Now it will be like trying to turn the titanic around.
25mcg patch is a very low dose, and you could try to negotiate a repeat scan in 3-6 months, but I think they will probably insist on the hysteroscopy if you want to keep your HRT.