Best topology for testing opnsense while using existing firewall?

[mannp]

Hi all

I have an aging system that I am looking to replace with opnsense.

I have 19.1.4 running on my dedicated hardware with dual lan.

How is best to run both firewalls in parallel so I can utilise my existing while I migrate functionality fully to opnsense please?

I have tried the existing (192.168.1.1) with a dmz of 192.168.1.254 and then set the wan of opnsense to 192.168.1.254 and the ip of opnsense to 192.168.2.1, but it seems that is wrong as opnsense cannot connect to the internet.

Thanks in advance for any help