For google rankings it's going to depend more on the update than the post-date.
For example, if you have a 10 paragraph post, and you add 1 paragraph and change the post date, I doubt you would regain the 1st position.
If you add 5 new paragraphs that add more detail or broader coverage to the subject and change the post date, you have a pretty good chance.
In both cases, you are not likely to get any kind of penalty for changing the post date.
On the other hand.... If you make a habit of REGULARLY updating post dates without providing any new value to the reader, then google will see this RECURRING behavior as spammy, and you'd probably end up on page 2 instead of position 2.
For the best ranking results I'd suggest you aim to update the article with a minimum of 20% new valuable content and update the post date. Possibly stick the word 'Updated: ' before the original title of the post as well.