updating a Url

You would change that bit by changing the permalink structure in Wordpress general settings
 
Yes, remember to 301 it. Link juice, authority, and such should flow to the newly changed URL.

Depending on case by case basis, your ranking for those subpages may drop, but will be back to normal within 1-3 weeks' time. In some cases, nothing drastic happens. I guess it depends on how Google values your sites.

Perhaps set your crawl rate (in Google Webmasters > Site Settings) to the highest for the time being as you do this.
 
Changing URL will impact everything and 301 redirect won't help you much. If the site is at least somehow ranking and gets some traffic, or if you already did some link building, then do not ever change URL structure.

I strongly advise not to do that.
 
Be careful with 301 redirections, it is a little bit risky for your SEO. In your case if you really want to change the structure of your URLs, it is your unique option.
 
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