To clarify the situation:

Per the features matrix/experience:
As a Gold, you are given 1 Fed, 1 KDF, 1 "universal".
As a Silver, you are given/allotted 1 Fed, 1 KDF.

You do not lose access to anything you may have earned as a Gold, however, in this case, Cryptic does actually yank the character slot.

So, as a gold, you have your 3 character slots, filled with 3 characters.
Downgrading to silver, you don't lose access to any of your 3 characters, but you only have 2 character slots.

2 silver slots + 2 bought slots from store = 4 slots. 3 existing characters + 1 new character = 4 characters. Perfect match.

This happened to me. Later, caught stories of ex-golds deleting a character then finding out they could not replace it. Therefore, confirmation that the only things (so far) that you actually lose in the downgrade from gold to silver is EC cap and free character slot...