Hi David,
First of all need to understand why you want to change the store because the data you are receiving from POSDM is for the same store. If you can give some details of your requirement, it will be easy to suggest a solution. If it is a business requirement due to any reason, i would suggest to change the store and system will derive the company code from site tables. But at the same time, since the site is in control record too as partner, it may not allow you to change the store.
We had used the same user-exit EXIT_SAPLWPUE_110 earlier to change the distribution channel to divert the online sales. Make sure you activate the enhancement WPUE002. Once activated, you will see system reading code in this exit during debugging.
You will have to try these things and update the thread with your findings.
I hope this will help.
Regards,
Amit