Redirecting URLs - Switching a site from Underscore to Hyphen?

When we first started using Drupal over 10 years ago the choice was made to use underscore separators (OLD) in URL aliases. I am now considering switching to hyphens (NEW), but want to retain the OLD URLs for search engine indexing and SEO purposes.

OLD: www.domain.com/this_is_some_content
OLD: www.domain.com/topic/this_is_some_content
OLD: www.domain.com/year/month/day/this_is_some_content

NEW: www.domain.com/this_is_some_content
NEW: www.domain.com/topic/this_is_some_content
NEW: www.domain.com/year/month/day/this_is_some_content

1. Hyphens seem to be preferred by Google. Am I doing a lot of work for little gain?
2. In your experience, what is the best way to do 301 redirects for the old URLs on a few thousand nodes and moving forward with the new separator format?