Transfem

If you switched back to T after being on E for a couple years, your body would just begin to redistribute fat back in masculine patterns. Just as the fat distribution changed when you went on E, it would change again when you switch to T.

Maybe this isn't relevant to you, but I personally thought I didn't want any breasts before I transitioned, and when I started E I was really worried about whether my breasts would be too big, whether I would hate having breasts, etc. That worry melted away after a few months on E, and after 6 - 8 months my perspective was completely opposite of where I started, that I really wanted more breast growth and that they are one of the best parts of my feminine body.

I would guess if you want a feminine body you might also find that after some time you realize you actually like having breasts, like me. Maybe not, though - but either way, you can't really have one without the other. Breast growth is a consequence of estrogen.

If it helps, lots of trans women have small breasts, and depending on your genetics you may not end up growing much. A common rule of thumb is that you will probably be a size smaller than your cis female relatives.

If your dominant hormone is estrogen, new fat will go to fem places. If your dominant hormone is testosterone, new fat will go to masc places. Old fat only goes away when either your body converts it to energy or it naturally breaks down after ~1.5 years.

If you stop taking estrogen and your dominant hormone becomes testosterone again, the fat you had placed while on E will go away when you burn it for energy or when the fat cell breaks down after ~1.5 years, whichever comes first. Except your boobs stay.

I got HRT through my doctor in the US. They gave me a big informed consent document with lots of details and warnings. A lot of the warnings are a bit fearmongery, like you might have a higher risk of this specific cancer, based on one small study decades ago.

Despite all that, there were only two things described as irreversible changes, breast growth, and infertility.

If you're an adult and no longer have an actively growing skeleton, and have taken precautions against the two above changes, you should be able to stop the HRT and go back if needed.

The changes are cumulative and gradual. You burn fat from everywhere, but deposit it based on your current hormones. With time fat will appear to migrate, and will migrate back if you stop the HRT.

Edit: specifying nationality is useful information when discussing supplied documentation.

It depends on your dominant hormone and how long you've been on it. If you're on E and continue to be on E you'll get most of the fat redistribution done by 2-4 years. If you're really skinny, you might not have obvious changes until you gain weight. Breast growth can take a long time, I've heard of sudden breast growth even with 10 years hrt.

For people that are unsatisfied with their fat redistribution, fat transfer surgery is an option for breast, hip, buttocks enlargement. Its usually about 10k USD for say a 2-3 cup size increase in the breasts, which is competitive with implants on the smaller side without needing to worry about silicone in your body. Plus side, if you gain weight, you get bigger boobs. You can take fat from wherever you don't like it being.

According to surgeons, silicone must be replaced every ten years and can have some pretty bad side effects.

From what I've seen of fat transfer surgeries, they also tend not to scar much, they redistribute the fat by sucking it out with needles.

I've been told by my doctor that fat redistribution is one of the slower changes to happen, and also one of the last things to change after you stop HRT. It's not permanent sadly, but it can take a long time after stopping HRT for the fat redistribution to reverse.