I had this situation a few years ago.
Swapping the bedroom/bathroom is usually a lot of expense and upheaval: with a standard bathroom replacement, you whip one out and stick another in within a day or two; but moving rooms, all the pipework, drains, electric wiring and ventilation need relaying, with floors and/or ceilings being ripped up meanwhile. Also, crucially, the external ventilation and drainage cost a HUGE amount to move or change, and in many cases can't actually be changed at all. If your tiny room is on the other side of the house away from the drains and in a place you couldn't dig a new one, it might be very difficult or even impossible.
Your age gap is quite small, so sharing a room might be OK. My boys are almost 5 years apart in age, and sharing only worked for a very few years, from 2+/7+ to 5/10. There were problems with different bedtimes, waking times and sleep patterns, and once my eldest hit adolescence, he needed privacy and it no longer felt appropriate for him to be sharing with a much younger child.
My eldest had the small room from 10-13/14, then they swapped so my youngest has had it from 9-13... IMO small children benefit from more space to play in, then teenagers need bigger rooms simply because they can't fit in tiny rooms, especially with a friend or two! Now I have two teens, the tiny room is a problem, and it's one of the reasons we're looking to move...