What happens to former friend requests when a person allows subscriptions to their Facebook profile? Do those people automatically become subscribers (provided that their requests are still pending)? Or is it only for the following requests?
Subscribers can get your public posts in their news feed. However, friend requests are a two-way interaction with more accessibility. The friend requests that were sent before enabling subscription would not be automatically added as subscribers. Subscriptions are only for the requests after enabling the feature.
It was my experience when I enabled subscriptions on my account that friend requests and subscriptions are completely separate once they're enabled. They were designed to be separate, with subscriptions being a way to filter what shows from a profile; a friend can subscribe and use that subscription to filter what gets into his feeds from your profile, and a non-friend (including pending requests) can use it to start showing all your public posts only in their feeds. However, one does not affect the other. You can message pending requests to ask them or instruct them to subscribe as described in this post, but there is no way to force them nor will it occur automatically. As far as I know, this remains the behavior on Facebook today.
Sources: Facebook help, Google search, personal experience
As of current Facebook's policy(and trying out myself):
"provided that their requests are still pending"
A Friend request is never automatically made into a subscription request or made as a subscriber. Regardless of the period the friend requests stays pending, they remains to be "pending friend requests". But meanwhile they them-self can subscribe to you.
"automatically become subscribers ?"
If you already allow subscribers, people who send friend requests that you ignore, delete or hide will become subscribers. People you unfriend will also become your subscribers. https://www.facebook.com/help/216792731707396/
Edit: Playing around with Facebook Help Center, I'm making my answer still more clear and readable.