QB Issue Resolution:

Changing the exchange rate or revaluing the currency rate won’t affect the previously posted transactions. The change is not retroactive and will affect the new transactions moving forward.

Resolution for Issue 'If i change the currency exchange rate from market rate to a fix rate will it affect the previous postings in quickbooks?' available: Yes (Solved).
Source: Intuit Community forum.

If i change the currency exchange rate from market rate to a fix rate will it affect the previous postings in quickbooks?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Changing the exchange
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Error Recovery for Changing the exchange [Canada]
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