I understand the table in the PHB that shows number of Cantrips Known at a specific level. I'm playing a Wizard and also using Hero Lab, which is pretty good about keeping you honest. However, in my last game I was able to purchase some spell scrolls from a vendor... the cheap ones I could afford were... Cantrips! So my question is this: If number of Cantrips known is 4 (per table in PHB pg 113) but through adventuring you're able to acquire more Cantrips via vendors, can those be swapped out for the 4 I know like memorized spells? I ask, because Hero Lab only puts lvl 1 spells and above in the spellbook... so there's no place to store acquired but not in use Cantrips.
You can't copy new cantrips into your spellbook.
DMG 200 says that spell scrolls are identical to spell books for the purposes of copying:
A wizard spell on a spell scroll can be copied just as spells in spellbooks can be copied.
However, PHB 114 states that you can only copy spells of first level or higher:
Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a level for which you have spell slots and if you can spare the time to decipher and copy it.
1st level or higher specifically excludes cantrips, so you can't swap them around like you can for higher level spells. In fact, you don't write cantrips into your spellbook at all.