Rather than creating a new challenge, can I just edit a past challenge to have new dates?

No, please do not do this!

If you simply edit the challenge Start and End Date fields, then everyone who was previously registered for this program will remain registered. Even if you edit the dates, someone who completed the program previously will still be counted as having completed the program. Instead, you want to set up a brand new challenge.

You can, however, duplicate past challenges.

Again, you may not repurpose an existing program with new dates or other elements after it has already been live and used by readers.