I've come up against this recently and I think a lot of imperative programmers will. For me, the basic pattern, starting from a career based on perl/php/java was this series of realizations:

- EECS circuit design is based on a conceit that gives chips a speed limit

- as a result, chips aren't getting much faster and that multi-cores are happening instead

- parallel programming will become more and more essential to make a buck as a programmer

- imperative languages aren't great for parallel programming due to mutable variables

- functional languages are better for parallel programming due to immutable variables

- functional languages are a lot closer to math concepts than imperative languages

- uh-oh

So for me it's been a fun few months of stumbling through self-directed study in stats, probability, and math. Bayesian probability from some articles by Yudkowsky. Fast, clumsy review of 0th-order, 1st-order, and zfc set theory. Anki decks out the wazoo. Now I'm plodding through Learn You A Haskell.

I would like to work more with actual mathematical proofs but I'm finding that tough to get into myself. It was fun paging through metamath for a few days, though.