I was grandfathered in at $500 on two accounts (both created in 2004), and have had premium on and off since 2010. I didn't lose my $50 stipend when I dropped premium, just as long as I did a weekly login. When I came back to premium last month, I still got my $500 on both accounts.
Here's my $50 stipend from last month, just before I decided to upgrade to Premium on this account again:
I can't promise that it works like that for everyone, but for my accounts, I've had no problem dropping premium, coming back and claiming the same stipends.