Capture Device ID for Posts that Do Not Have User IDs - Advise Needed

I'm planning to build an Android and iPhone app that allow users to post questions. However, users won't necessarily have to register to post their questions. They can simply post a question straight right after installing the app. On the other hand, users can register and login to post their questions. The advantage of registering is users will be able to keep history of their posts and the like.

I want to allow non-registered users the option to register and get all their posts that were posted before registration to be moved into their registered accounts atomically. For example, if a user downloads the app and asks 3 questions before registration. After the third question, the user registers, logs in, and continues to post questions. I want the 3 questions that were posted before registration to be part of/moved to the registered account so when user logs in, the user can see the 3 questions in the logged in account. Is this possible?

I thought of capturing device ID when a user posts a question without registration and then when a user signs up, I will capture the signed up user's Device ID and if the signed up device ID matches the device ID of the posts that were posted without registration, I'll shift all of these posts into the signed up account because the signed up account has device ID that match posts device ID. Does it sound like a good idea?

I'm currently brainstorming the workflow of the app. I don't have codes on hand yet.