I never found a way to change the number of voices in the poly object other than changing the creation argument. eg [poly 8 1] to [poly 1 1]
Here is an abstraction to address this

poly-dynamic.zip updated

I've just updated the above poly-dynamic to pass through the clear and stop messages to poly

this will also be a feature for [poly] in the next pd release https://github.com/pure-data/pure-data/pull/661

Damn too late again