The verb “taper” is defined in dictionaries as:
So, this word is often used with the adverb “down”. I am searching for a word that means To become gradually wider or thicker toward one end. Is it possible to use the expression “taper up” for the meaning?
If you put an opposite word of an adverb, it does not convert the word into the opposite meaning.
slow down cannot be turned into slow up to mean faster!
The word you may use is flared which means widening up. Though it has limited usage and may not suit to your concern.