I'm writing an essay (MLA style) for class and I am quoting a whole sentence within my sentence. 

For example:
They published a “manifesto” proclaiming that “[a]ll are welcome to walk through our doors and speak."

In the article the word 'all' is capitalized because it is the beginning of the sentence. 
However, when quoting the sentence do I keep it capitalized, can I just switch it to lowercase, or do I have it right as is?