I changed maybe 1-2 sizes in the whole 300s. At 330 I was wearing size 30 jeans. By 320 or so size 28s worked.
Then, when I was about 300 I made it into 26s. Since then the progress has been pretty fast.
So: last August, bought my first 26s.
late October/early November: size 24 jeans, tight but celebrat-y!
January: still wearing my size 24 jeans around, bought my first pair of 20s! What??
March/April: uh, the 18s fit?
May: the 18s I just bought last month are no longer fitted. They are loose. I have two pairs of stretchy-waisted size 16s that are in regular rotation.
So, yes. Progress is slow at first but it IS progress. And if you keep going (sailing by), you will wake up one day to realize that you have no idea how on earth you ended up with so many clothes you can't wear.
You can totally do this. Slow and steady wins.