The information you need can be found in two pages Apple publishes:
Apple One - Apple
What happens to your iCloud storage when you sign up for Apple One - Apple Support
The 200GB space you mention is from the second tier offer for Apple one, which is called Family. The tier above this one is called Premier and offers a 2TB storage space. Both are sharable with a family group.
All Apple One plans come with Apple Music (and sharable in the Family and Premier plans).
If you subscribe to the Family Apple One plan (with 200GB storage space) and have an existing 2TB plan you get to keep the 2TB plan - which continues to be billed. This is described in the second liked document above:
If the iCloud storage included in Apple One is less than your current plan - You will keep both your current iCloud+ plan and the iCloud storage in Apple One.