TL;DR: Regarding how you make the switch, I'd suggest going with your gut feeling on what you'd consider the most sustainable way of transitioning.

Some folks need to change things slow and steady and just take some time to make small changes and adapt. Others simply go balls to the wall, change overnight, and then proceed to power through the first couple of weeks until it becomes habitual. Personally, I'm the latter of those two types. Going slow would end up boring me before I've made all the changes I want to make. It also makes it possible for me to turn it into a bit of a game or challenge for me in some way (in contrast to going slow, which feels like less of a challenge).

The biggest pitfall with this tactic is probably the scenario where you deplete your willpower and slide back into old habits. Another one that I noticed when I went cold turkey from omni to PBD was that after a few weeks I got very fatigued. I just wrote it off to my body finally adapting to the new food intake and saw it as a temporary state of affairs. It lasted a few weeks, with me just feeling tired, barely having the strength or endurance I used to have while exercising, and muscles recovering much slower from said exercise. Just powering through that ultimately paid off and after a while I was reaping all sorts of benefits from switching to a PBD.

Anyway, just my experience. Hopefully it helps you in figuring out how to make the change. Just keep in mind there's not one single way that is the best way to do this, and you don't have to stick to whatever you choose initially. If cold turkey was too fast then just change tactics and opt for the slow and steady approach, or vice versa, or whatever else you come up with.

Since you're making this change with the both of you it might take some mixing and matching on finding a way to transition so that you both get what you want out of it.

Good luck!