Now as for rewriting itself, yes, do not be afraid of it. Your first chapters are essentially your business card. The better they are, the more will stick out with you. Readers have no patience. Getting better later on will never be rewarded. You either have them hooked or lose them.
Let’s start with the reaction part. Generally, readers do not react. Firstly, because they would never notice a rewrite of earlier chapters unless you do a complete retcon. Secondly, because readers who reread your works periodically are exceedingly rare to the point of being negligible. I have some of them, but as said, they are rare.
