Question about synonym expansion at index time and synonym changes

I'm looking at ES as a poc and was wondering if anyone could fill me in
with best practices or common pitfalls with synonym management at index
time. From what I gather, ES will not automatically update already indexed
documents when you've modified the synonyms config files. Assume a
sharded, replica environment.

What is the normal/preferred procedure for adding a new synonym that you
want ran at index time? Alter config file on all servers, rolling changes?
When you update the synonym config, do you need to restart, flush, reload?
What are the implications in doing so? (performance degradation, node
Can I trigger a re-index of the affected documents or should I simply
re-feed the document from source? (I'm assuming changes are not
automatically updated)

I've read through the documentation related to the subject matter, but I'm
looking for real life feedback from deployments, operations. Thanks