Having dug deeply into the Michigan Treasury website, I see no provision that would leave the residence taxable value as it was before the old residence burned. There are exemptions for those in the poverty income level, but my review of all the government forms for real property disclosed no exclusion due to disasters such as a fire. I did read Proposal A as you can below. With a newly build structure valued at $123,000, there is no provision that would allow it to be given a taxable value of $43,000. There is no mention of hardship exclusions for fire or disaster....only for the poverty exemption.
The land valuation is protected under Proposal A and you should make sure that the land value was not reassessed, but not the new construction as you can see from the direct quotation below:
"Beginning in CY 1995, Michigan property taxes are levied on taxable value. A constitutional amendment requires that the taxable value of a residence or business cannot increase in any one year by more than 5 percent or the rate of inflation, whichever is less (excluding the value of new construction). "
http://www.michigan.gov/documents/628f_2685_7.pdf Notice by Owner of Property Incorrectly Reported or Omitted From Assessment Roll
http://www.michigan.gov/taxes/0,1607,7-238-43535_43539---,00.html Principal Residence Exemption Program
http://www.michigan.gov/documents/2368f_2605_7.pdf Homeowner's Principal Residence Exemption Affidavit
I hope you review the above website when you have the opportunity. The information is enlightening, but offers no relief for you on the taxable valuation.