Controlling lock up in a 904 lock up

I have a 904 lock up in my Scamp (or 999/998, it's an early '80s model) and I don't like how soon it locks up the converter. It locks at 35mph and doesn't unlock until I've slowed down to around 15mph. No throttle response makes it unlock once it has locked at 35mph. Now of course if I give it enough gas it will kick down into second, but once it shifts into 3rd, it locks up at the same time.

I'm just wondering if there's a way to change when the lock up occurs. I'd like it to lock up more around 45mph. Is this possible?