You start out with a list of words containing at least four letters and at most eight letters like this:

aDage
bAlky
aNnum
cAnal
eErie

Vertical words used:
danae

Each word must have the same number of letters.
The number of words must match the number of letters in each word.
The list of words must spell out a word vertically, up or down, using the letters in the same position of each word.
Highlight the vertical word using caps,
Add the new vertical word to the list of used vertical words.

Then you can do one of two things.

1. Swap two and only two words around to spell at least one word vertically up or down using the letters in the same position of each word where the vertical word, regardless of direction, has not been used previously for the list of words.

2. Substitute another word with the same number of letters for one already there that spells at least one word vertically up or down using the letters in the same position of each word - but this must be done in place, a direct substitution and you can't move any of the other words. The vertical word must be unused for the list of words before the substitution but resets the list of used vertical words.