I'm getting to like your SO more all the time.
It is good to make plans but life doesn't always work out the way you'd like, especially when dealing with chickens long-range. I wish you luck in that aspect. But I'll propose an alternative for you to consider and tell you why.
Often integration goes a lot better than you'd think reading the stories on here. Even if you don't do the things we often suggest on here to improve your odds it often goes really well. But sometimes there are problems. Just because one person is successful doing something doesn't mean it will work for you, just like one person's failure doesn't mean you will fail. They are living animals and you don't know what will happen.
Chickens are social animals and like having other chickens around. That does not mean they are going to die if they are alone but the preference is to have company. So if you bring in new chicks with a single hen she has a tendency to join them. Sometimes no big deal, but sometimes she can be a dangerous bully when the chicks invade her personal space. It's not logical, she is the one invading their personal space but gets mad at them for invading hers. What typically happens when you have a few older chickens and integrate young ones is that the young ones form a separate sub-flock until they mature enough to join the main flock, usually about the time they start to lay. I suggest you use that so she is never lonely.
The spring that you are planning on butchering the others, bring in your BR chicks. Raise them and go through a normal integration, you will probably need similar facilities and room whether it is one adult or several. Hopefully you have room. When those pullets start laying and can give her company, butcher the others. They are not going to be laying much anyway, the pullets will give you eggs, and if you butcher them before they start to molt you don't have all those pin feathers to deal with if you pluck instead of skin.
I don't know if you have the facilities to manage this seamless transition. A critical part is how much room you have, in the coop and in the outside space. I don't know how important eggs are to you either. It's not that they just shut down laying all at once, it is a gradual decline over the years. You may go a long time with them laying very few eggs. You or your SO may object to buying all that feed and getting very little return in eggs. As you gain experience you may find you want to do this on a regular basis, every two or three years, instead of waiting until they absolutely stop laying. As I said, plans change. You need to be flexible. But it is good to have a plan.
