title_body: What is a constructed variety of a natural language called? What would a conlang created to be an alternate version of an already existing natural language be classified as?
  
  For example, a version of English that could have been spoken in an alternate steampunk-era Victorian England, or a conlang of the Finnish language in the year 2349. To be clear, I'm assuming that the conlang would be an actual, dedicated one, and not just a few "alternate" words thrown in for flavor; the examples I picked were just the first ones that came to mind.
  
  (Bonus: is there a classification for a conlang created to be a conlang of a conlang in this same way?)
upvoted_answer: That would be an a posteriori conlang, in contrast with an a priori one. Wikipedia. The process of changing a language through time is called diachronic conlanging. 
  
  An example of a very well-done a posteriori conlang would be Carisitt which has been developed as if it was a natural language deriving from PIE (that is, it is also an example of diachronic conlanging). Examples of an a priori conlang are abundant, for instance all of Tolkien’s works.