Mikrotik, How to change the source address of one locale to the address of another?

Question

Mikrotik

Good day, this is the problem I'm facing.
There is a microtic rb951, it has 2 interfaces for two local networks (100 and 123) and routing is configured between them, a simple diagram is below.

We recently purchased a firewall through which users from 123 networks work through it on a web resource. The problem arose in the fact that there was a need to work on this resource and people from the 100th subnet, but this subnet is already occupied by another institution, so the upstream gateway drops all requests from my 100th subnet.
Now it is physically impossible to reconnect people or set up vlans, there are 2 options either to reconfigure the entire 100th subnet to the 200th one, or somehow raise the "internal" nat, so to speak, so that when a request is made to 192.168.123.50, computers from the 100th network access through one address, is this possible?

Answer 1

Set up a regular nat masquerade in the srcnat chain, where src add is 192.168.100.0/24 and dst address is 192.168.123.50. Then all requests to 192.168.123.50 will go with the address that is assigned to the Mikrotik interface looking at the network 192.168.123.0/24