Convert
I converted Preferred to Rewards +.
I agree it's really only a minor change in product. But one of the factors that pushed me over the edge toward the product change which may not be huge in your case was the 10k typ annual rebate on redemptions.
If you don't have a large aggregate typ account balance then it doesn't really matter which card you have unless you do 1.00 transactions almost every day. But 10k back was a pretty compelling reason.
In the conversion which was solicited by citi my points remained intact, they did not expire with product change, the account number did change and it took maybe 30 days. Because the preferred card was pretty much useless to me I went for the conversion.