Well, the simplicity depends on two things
1. Do you have any other Guest posts?
2. Do you want these all on one account, or one for each member?
If the answer to 1 is No, and the answer to 2 is one account, then you've made it very simple. Just create a new user, get the user id, and update poster_id to the new user where it's currently 1.
If the answer to 1 is yes and the answer to 2 is one account, then you do something similar, but use the poster column in the posts table instead
If the answer to 1 is no, and the answer to 2 is multiple accounts, then you do just like you would above, except you only assign one poster to one account.
If the answer to 1 is yes, and the answer to 2 is multiple accounts, then it's the same as the two above (not the first one) pretty much.
Just tell me what your situation is and I'll write out a query