Permalink Chnage Problem

I’m helping a friend and when they setup their account had permalinks setup with a ‘Y’ at the start so it was setup like /Y%year%/%monthnum%/%postname%/

When I changed it to the correct structure (/%year%/%monthnum%/%postname%/) then the posts redirected properly but the tag pages didn’t so the URL https://www.domain.com/blog/Ytag/cars/ just shows a 404 error.

I tried putting a rewrite rule in the htaccess file to fix this but it didn’t seem to work. The rule was: RewriteRule blog/Ytag/(.*) blog/tag/$1 [R=301]

Any thoughts on what I’m doing wrong?