Since you're here on the web, you can find the manufacturer at http://www.centuryproducts.com They're the only ones who can properly answer this question. If you're missing the owner's manual, you might want to see if they can send you a copy.
Actually, I am happy to properly answer your question
With either the Century or the Century/Graco EZ LATCH kit, you may continue to use LATCH with your combination seat after you remove the harness. Of course, that assumes your Breverra is made after 9/1/97.
This will not necessarily improve the safety, but it will help keep the seat stable when the child is getting in or out, and when the seat is unoccupied.
You may also cease to use the LATCH attachments altogether if you prefer.
This subject is still a topic of some interest. The current best practice is to rely on the manufacturer's instructions. It is worthwhile to note that Cosco and Evenflo do not permit use of their LATCH retrofit kit with combination seats when the harness is removed and the seat is used as a belt positioning booster. The Century recommendation is in the LATCH kit manual. It is on Page 8 for both the Century #4025 kit and the Century/Graco #8405 kit.
As for tethers, my Century Next Step does allow the tether to be used for both modes (harness and belt-positioning booster). This may vary for your Breverra. If you don't have your manual and Century can't help, let me know and I will look it up for you if I can locate my CD-ROM
[This message has been edited by caviller (edited 03-04-2002).]