https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-09-12/08-covid-priddy-508.pdf
Moderna tested the newest XBB.1.5 vaccine in human trials. Here you can see the reactogenicity data for it compared to the original vaccine and the bivalent booster in adults.
In any case, at this point it is entirely reasonable to move to a flu vaccine model- we have more than enough human safety data. We've now had several blinded placebo RCTs for the coronavirus vaccines. Billions of doses have been administered. The first human received the first dose 3 and half years ago.
The new Moderna vaccines have the same dose as the bivalent booster- 50 micrograms- versus the 100 micrograms of the original vaccine. They are in monovalent form like the original vaccine. The only thing that has changed is the variant they are targeting is XBB.1.5 instead of BA.5 (the bivalent booster) and Wuhan‐Hu‐1 (the original vaccine).