How is my application’s safety certification maintained or re-asserted after converting to C?

Probably the best analogy is recompiling the system with a new compiler. Clearly you would have to rerun all your tests, but presuming you translate any Ada unit tests/drivers as well, you should be able to achieve the same level of MC/DC coverage achieved before the translation. Fault analysis, etc. should be identical. All of the comments in the code are preserved, and the code component names remain essentially the same, meaning that documentation should not need to be altered in any significant way. Given the ability to translate the tests/drivers, achieving adequate MC/DC coverage should not need redoing. Ada-C/C++ Changer is very mature. The Ada 95 front end used to drive the translation has been in active use for over 10 years, in many safety-critical systems, because of its use as part of the Aonix and Green Hills compilers.