Yes, the firearm is registered as an SBR, and you can change out the “brace” device or stock for a different brace or stock. You do not need to contact ATF/NFA because changing the brace/stock does not change the configuration of the SBR. However, if the length of the firearm has changed you will need to notify the NFA Division.
Final Note: The SBR is not “registered” until an approved Form 1 is received from NFA. The brace cannot be substituted/replaced until that time.