Thanks. I'm not sure. I've heard it really depend who is willing to do it. Some jewelers won't touch a diamond they didn't sell, and some insist they set it if the ering was purchased from them.
Start with the setting manufacturer/seller. There's a pretty good chance that the diamond seller won't agree to do it anyway but it really depends on the details. Sometimes it's easy, sometimes it's impossible.
Make sure you understand who is taking liability risks and how it affects your warranty on the mounting, if any. These vary quite a bit but usually, it's all on you. A certain amount of this can be passed off to an insurance company but the first step is to understand the rules.
Any setter, no matter who it is, will want to look at both the stone and the mounting before fully committing to the job. Sometimes the difficulties aren't obvious. Make sure you have this evaluation done while you're still within your return period for both parts, just in case you need to change one or both to make it work together and get you what you want. Usually the return policy ends as soon as a anyone takes a tool to it to set a stone or size it so get this question resolved before any work starts.