...

I have tried all possible word combinations that allow a one-letter change...

None make sense, unless you're shortening pollen to poll, or collagen to coll.

In fact the only related thing was the plant 'dill'. which isn't a capsule...

For the thread sake, I shall assume the answer is 'dill' :3

Dill

When you go shopping, you can pay at the ___

Till perhaps?

Something that is commonly used for walls or flooring

Bill

Hint: you can play with a _____.

ball?

Another way of saying Autumn..

(we are running out of ways, maybe we should change the starting word soon?)

Fall

Market

Mall

Hint - a building consisting largely of a principal room

Hall...?

I'm changing the word because I feel we're in an endless loop <object class="emojione" data="https://resources.enjin.com/1489581540/themes/core/images/emojione/svg/2639.svg?0" type="image/svg+xml" standby=":(">:(</object>

Try varying which letter in the word you change so this doesn't happen again <object class="emojione" data="https://resources.enjin.com/1489581540/themes/core/images/emojione/svg/1f609.svg?0" type="image/svg+xml" standby=";-)">;-)</object>

New Word: Dust

Dust

Hint: I ___ Build it

Must

Hint: The room had a ____y smell to it.

Musty

The car is

Close, but no.