Question about endometrial cancer

I had a uterine biopsy two weeks ago which showed grade 1 cancerous cells. I haven’t met with the gyn oncologist yet. When my regular gyn biopsied me and examined me, she originally said cancer was very unlikely and later apologized for saying that. Of course I 100% don’t hold that against her, I trust her very much, but when she says its the least aggressive type, least likely to have spread already, most likely I’ll be able to keep my ovaries, I do think to myself, is that likely?

I keep reading stories how people think the cancer is mild until they have a hysterectomy and then they find out its some other type.

I know we wont know the full picture til hysterectomy, but my question is, if they found grade 1 cells, how likely is it to have grade 2 or 3 cells deeper in the uterus?

I feel like I’m coasting on “best case scenario” but I feel like I should mentally prepare for all different outcomes.