“It's been downgraded to a tropical storm.”

You're talking to your mother about a hurricane that is headed toward you. You heard on the news that the strength of the hurricane has dropped, so it's not 

It's been downgraded to a tropical storm.

(be) downgraded to (something)

For something to be "downgraded" means that someone has changed it from a higher level to a lower level. You use this when you're talking about the level or classification of something. For exmple:

Their credit rating has been downgraded.

Are you able to downgrade your account to a lower level?

Use "from" with "downgrade" to expres the original level and "to" for the new lower level.

a tropical storm

A "tropical storm" is the same as a hurricane or a typhoon, but with less power and lower winds.