The BTUH capacity IS going to change. It'll need to be a lot smaller, to not short cycle.
If you are changing the radiator output from 240 BTUH per square foot to 170 BTUH sq ft, the boiler will be considerably oversized.
Now, instead of a few gallons of water to heat up, you'll have "how much" water to heat up? Can the hangers support that extra water? Will the slower response time matter?
With the extra water, do you think the cost to run will go up or down?