Hello We have a client that needs to have more flexibility on the blog pages, they are using a highly customized 1.0 theme so moving all the themes to 2.0 is too much work for them They need us to move only the blogs to 2.0 and create some blocks
I wanted to know if this can create any type of issue
You should be able to create new 2.0 json templates to be used solely on blog articles.
However, due to some weirdness on how these new templates work and were designed, you won't be able to assign that same template to multiple articles unless you want them all to share the same block content/data that get's stored in the json template file. In other words, If you assign the same template to 2 articles, and edit a block section in one article, you'll notice that the second article's content will be replaced with the newer content from the first article because they both share the same json file.
If you want the client to be able to edit these article block content via the theme editor, then you would have to take the duplicate a new json template file, per new article approach, from here on out.
The alternative would be to setup the section blocks to feed off from a 3rd party custom fields app. This approach is not the best because the user would have to leave the theme editor to be able to input the block's content via the 3rd party app. It's basically all a mess and very disheartening.