Flying TP is not a downgrade. I went from a jet FO position to TP (DHC8) FO due aircraft reduction at base. I really didn't want to do it but had no real sensible choice. Guess what ? I thought it was great ! You could do things you couldn't with a jet. Loads of good experience with weather and fun handling.
I understand the ATR is a good machine.
If job security is the issue, then as others have said, grab the ATR with both hands and don't let go.
I guess you have to work 3 months notice so you should get the 500hrs plus the TR is good for 5 years anyway. Do bear in mind that the 500hrs is not so important for your new employer...what they will like is the fact that you have the 737 TR so when you do slide across you will be an accountants dream.
Good luck, and stop thinking it's a downgrade. You'll be better for the experience. I was.