Do I need to do anything about lots of 404s pointing to the old pages?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

• 
  miksmusic: A quick question.. My Web site went through a major redesign recently (from a blog to web shop). As a result, Webmaster Tools report lots of 404s pointing to the old pages. Do I need to do anything about it? If yes, would would you recommend? Thanks!
• 
  W.E. Jonk: Yes. If you change your CMS it is common that URL's change too. If you can I would serve the same URL's, if you cannot you should set up 301's to the new URL. ;
  
  Even if you think you have set up all the 301's it is normal that you missed a few. Therefore keep an eye on GWT for 404's and iterate over it. Furthermore GWT tends to report those 404's even when you solved them. In GWT you can mark them as solved. Although this doesn't affect Googlebot or anything on Google it does set other (404) errors in GWT higher in the list, which, to me, makes it a little easier to manage...
• 
  W.E. Jonk: From the expert panel in this weeks SEO Questions hangout on air on 01:32:18 into the YouTube video: https://dumbseoquestions.com/q/do_i_need_to_do_anything_about_lots_of_404s_pointing_to_the_old_pages +miksmusic
• 
  miksmusic: Thanks! Much appreciated.