You need to figure out why your teeth are shifting first before considering any new dental treatment. I would not recommend replacing the crowns on the front teeth because they will be very wide and bulky to close that much space. If you have bone loss causing the issue, the teeth may need to be removed. If you have excessive force on the teeth moving them, that needs to be addressed and orthodontic treatment done to move the teeth back to their proper position. Teeth can separate and bulk forward from long term use of CPAP masks. If that's the case, you should try to switch to a cannula type mask or an oral sleep appliance to prevent further shifting of the teeth.