Going from co-advisors to just one advisor I'm 2.5 years in to a 4 year PhD with possibility of a 5th year of funding. I started with one advisor, but research interests started shifting, so we brought on a co-advisor. My research interests have continued to shift in the new co-advisor's direction. The research components that the original advisor can guide me on are hitting speed bump after speed bump and never really getting off the ground. I've been making steady progress with the research components of the new advisor. I want to have the "new" co-advisor  (not so new by now) just be my main advisor, and have my original advisor just serve as a committee member. Feels like a cold move, seeing as he was the one that brought me on in the first place. But he recently mentioned he's no longer sure how to advise me anymore, so I think he might be thinking the same thing.

I worry I'm too far into my program to make such a move. My comprehensive exams are coming up this semester, which complicates the timing of this as well. I want to suggest this move to my co-advisor in a way that is tactful and won't cause waves. But I'm not sure how to go about it - you can't un-ring a bell.

My questions are: 


How unusual is it to go from two advisors to one after 2.5 years, and am I better off making this change or doing the best I can with the situation I'm in? 
If making this change is better, do you have any advice with regards to how to do it so that I don't make a mess? 


I am not being directly funded by either advisor, like with a grant. Funding comes through university assistantships. Additionally, keeping the original advisor on in a co-supervisory role is not an acceptable solution, because the original advisor is adamant about research approaches that don't work, and - at this point - also fall outside my interests.
I think the optimal answer depends mostly on your situation and university. However, I try to give you some general advice. From what I have seen in your question, you might already be one step to far into the decision. I would rather start an open discussion with your first advisor and express your concern. All further steps, such as switching advisors, can be the outcome of your discussion, but there might be different and better solutions.

It is quite normal that research interests shift after some time into your PhD. Since you are making progress, you see which ideas are working out and which don't. You will get to know new topics and new questions will arise. This will have an influence on your research, of course.

Your advisor seems to see that, too:


  But he recently mentioned he's no longer sure how to advise me anymore, so I think he might be thinking the same thing.


By expressing his concern, he already shows some awareness of this issue. You should try to talk to him and find a solution together. I would not approach him like "I want to replace you with advisor Y". This can come across as very rude since this is on a personal level. Start a discussion that focuses on your research. Talk about your concerns that your interest have drifted apart and try to come up with possible solutions. When you are further into discussion you could also include the co-advisor. As already mentioned, switching advisors might be one solution but it might not be the best one.

The more you integrate your advisors into the discussion, the less conflicts I would expect. Do not push your decisions onto them as this could really affect your current position. Talk to them and find a solution together.