Is it possible To change baud rate at runtime?

[HenrikOlsson]

Absolutely. But you can't use DEFINE because it's a compile time thing.
You can skip the DEFINE altogether and do it manually all the way, what to set the registers to depends on the oscillator speed. For example, setting 9600 baud when running at 8MHz:

RCSTA = $90
TXSTA = $20
SPBRG = 51
SPBRGH = 0
BAUDCON.3 = 1

/Henrik.