Permalinks – Post name vs Default & a merger

Good Morning,

I hope that I can explain things well enough.

I merged two sites together a few weeks ago for a friend. On the old site we setup a 301 redirect.

Everything is working fine and everything is linked up perfectly.

She would now like to change the permalinks on the new site.

I believe she is using the month day setting currently on the site.

This is my question. If I set the new permalink setting on the new site to post name will the old site 301 direct links be broken or does wordpress focus 1st on the default setting which to my understanding is always set and always stays the same?

I want to make sure that when I do the change to the permalinks that I don’t break the redirect for her.

Thanks so much for your time and knowledge.

Blessings
T