ModbusMaster.h - changing node
Maybe this question is too old and the API has changed, so my answer is reference.

The ModbusMaster constructor does not accept parameters.

From reading the source code, the method begin(uint8_t slave, Stream &amp;serial) just initializes variables and can be called again with a new slave value without the need to destroy the ModbusMaster object.

Reference:

[1] https://github.com/4-20ma/ModbusMaster/blob/master/src/ModbusMaster.cpp, line 61.