Yes, you can have different baudrate(s) on different UARTs and yes you can change it at any time you want. But, as the posts in the other thread says, to change it you can't use PBPs DEFINE HSER_BAUD because that's a compile time directive and not a runtime command.

If you want to set or change the baudrate at runtime you need to set or change the registers manually. BAUDCON, SPBRG, SPBRGH most of the time. And remember that the register names are different for the second UART - BAUDCON2, SPBRG2 etc.

Without seeing your code we can't say why what you did didn't work.