All that is changing is the little piece of non-infectious and benign spike protein that your own ribosomes are producing, so that your immune system can identify it later. The mRNA sequence itself is only present for a couple days at most before it degrades and you no longer produce the spike proteins.

And yes, even under the emergency use authorization, each new mRNA sequence is tested for safety and efficacy in animals and humans before release.

The sequence could encode for millions of bits of viruses and other things that would all be non-harmful. The preservatives and other ingredients are what have caused serious events such as myocarditis, anaphylaxis, etc.

I will try to find a meta analysis of a bunch of mRNA vaccines or a study on mRNA technology when I can.

Basically though, it is impossible that changing the spike protein is gonna change anything except what your immune system learns.