is it possible to change mac layer of xbee pro modules?

XBee & XBee Gateway XBee Programamble (S2C & 900 HP)
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2

which XBee module are you referring to?

3

XBee-PRO ZigBee SMT
(PN:XBP24CZ7RISB003) &
XBee-PRO ZigBee Through-Hole
(PN:XBP24CZ7PIT-004)
I want to implement beacon enabled mode of 802.15.4. Is this possble?

4

No, that is not an 802.15.4 product. That is a Zigbee Enabled product which has 2nd processor on it. It is referred to as a Programmable XBee. Ember does not offer an 802.15.4 Stack to work from.