It would have taken many years to get the new technology approved, but we skipped a lot of that due to the pandemic. Now that we have lots of safety data it is easier for them to get new vaccines approved.
It probably helps that there's now a well develiped framework for mRNA and adenovirus vector vaccine approval, but ultimately any new antigen could certainly be unsafe and future candidates are going to require every bit as much testing and won't get most of the expeditiory benefits the first generation of COVID vaccines experienced.