How to Change Permalinks Without Breaking Anything?

I want to add sub categories because I feel that it would improve relevance, particularly when it comes to showing related posts. It just makes sense to have posts about acne and toothpaste, for example, in the same broad category yet in different, more focused sub categories. Every large site I've visited does this. Topical relevance would be much improved.

1. At the moment I have:

domain.com/health-beauty/postname

I want to change it to:

domain.com/health-beauty/skin-care/postname

domain.com/health-beauty/oral-care/postname

How can I change this without affecting rankings and backlinks built to domain.com/health-beauty/postname ?

2. How relevant do I have to be with the sub categories? I don't want to be as focused as Amazon, but is it okay to have something like posts about coffee makers in the same sub category as washing machines because they're both appliances? /home-garden/appliances would be the structure.

3. Is using tags instead of sub categories an option? I can see how it would be good for interlinking related posts automatically, but not so much the structure of a site.

I am only equipped to answer #1.
The solution is to 301 redirect the old URLS to the new ones.
http://www.wpexplorer.com/change-permalinks-wordpress/

Anyone got anything else to add?

johnsampson said:

Anyone got anything else to add?

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here you go, many tutorials.

dhia27 said:

here you go, many http://lmgtfy.com/?q=How+to+Change+Permalinks+Without+Breaking+Anything%3F.

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I was talking more about questions 2 and 3.

johnsampson said:

2. How relevant do I have to be with the sub categories? I don't want to be as focused as Amazon, but is it okay to have something like posts about coffee makers in the same sub category as washing machines because they're both appliances? /home-garden/appliances would be the structure.

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The more relevant the better, but try to keep URL lengths to a minimum. What you don't want is an URL like domain.com/home-garden/appliances/kitchen/coffeemakers/brandco-model2

Keep it relevant, but short and with the most important keywords... domain.com/kitchen/coffemakers/brandco-model2

3. Is using tags instead of sub categories an option? I can see how it would be good for interlinking related posts automatically, but not so much the structure of a site.

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No, don't use tags for that. Google wants to see the hierarchical site structure here, and tags don't give you that.