In reply to Victor Miller.

It won't affect them at all.

Even when you open them in Gutenberg for the first time, the content will be in a Classic block: a block that is (technically speaking) the absence of a block, since it is used in the editor for any content that is not between block delimiters. The block is basically an embedded version of the TinyMCE editor box in the Classic Editor, and it acts pretty much the same. You can choose to convert the Classic block into standard Gutenberg blocks, or you can just leave it as-is.

Side note: the Convert to Blocks option currently strips out some formatting (inline color styles, for example) that it should not. There are some open issues and PRs on the GitHub project regarding this:

https://github.com/WordPress/gutenberg/issues/6102
https://github.com/WordPress/gutenberg/issues/6878
https://github.com/WordPress/gutenberg/issues/7604
https://github.com/WordPress/gutenberg/pull/7995
https://github.com/WordPress/gutenberg/issues/8450

Luckily, you can just leave the content in the Classic block and not convert it to standard blocks until these issues are fixed. Or you could just never convert the content. If you are not going to be editing those older posts, then there is not really a reason to mess with them unless you just want to do it for fun like I did with my old posts back when I first installed Gutenberg on my website several months ago. :P