@jocke4u I run 2 RF gateways and it's not a problem. When you 'migrate' a node to a new gateway make sure to run the mysensors clear eeprom sketch first to get rid of the old routing table or you might have issues.
@Tommas there can only be one gateway per MySensors network. So you'll need to put the nodes on different networks.
See https://www.mysensors.org/about/network for more info on how nodes, gateways and networks work.
How to set change the channel depends on which transport you are using. Look for the channel setting in MyConfig.h. All nodes in one MySensors network must use the same channel.
I will answer myself after some testing done today:
How can I change network id for gateway (mysgw)?
It is possible to change MY_RFM69_NETWORKID for mysgw by exporting CPPFLAGS variable with the option set before running ./configure:
./configure --my-gateway=serial --my-transport=rfm69 --my-rfm69-frequency=868 --my-is-rfm69hw --my-serial-is-pty --my-serial-port=/dev/ttyUSB-MysGW --my-serial-groupname=dialout
Is it possible to create another RFM69 network on new hardware, just for the test, while my current RFM69 network keeps running on current gateway/controller?
As soon as I started another RFM69 gateway (mysgw) on new hw, even with different network id (101), most of the sensors connected to my other (production) gateway running on old hw with default network id (100) stopped communicating... so it seems it is not possible to run multiple RFM69 gateways in the same range (on the same freq).