How can you avoid plagiarism when you're writing your PhD thesis?
If you're taking chunks of other people's text and changing words to make it look different, this is still plagiarism; you're taking someone else's work and altering it to make it look like your own, but without any thought beyond finding pseudonyms.
A good reader will spot this, because your style will change depending on the work you alter. It's also easy to spot the weird word choices that result from over use of a thesaurus. Sometimes you can even see the formatting of the text change because it's been pasted from another source...
You should never copy-paste someone else's text and try to alter it. Instead, you need to be able to describe and comment on the work without having the text in front of you as you type.
The key is not to think about changing other people's words, but about developing your own perspective.
This will be difficult if you're focused on individual papers in isolation, but it gets much easier when you think about what's happening in the field collectively.
For example, if your literature review has a section talking about techniques for measuring x, you don't just take the first paper you find and copy/ adjust random sentences. First you need to have a knowledge of the various techniques, how they differ and their relative strengths and limitations. This knowledge is the foundation of your own original writing.
You also need a knowledge of the collective trends in the literature or what's happening in the field. You might notice, for example, that a lot of papers disagree on a particular point, or that most studies tend to rely on the same theories or assumptions, or that the way things are done is changing. These observations of the literature as a whole help to put individual papers into some kind of context, while also allowing you to give your own perspective.
What have you noticed about the literature? What do you think is important or interesting? Start with this, then you can use individual papers as examples to illustrate the point you want to make.
If you base your writing on what you want to say—on your knowledge and your observations—plagiarism wont be a problem.
See also
How to write a compelling literature review
Academic Writing: Context is everything
