Hello. I have a Mikrotik hardware with factory firmeware - 6.49.15, Is there a way to downgrade it to 6.48?

No, factory versions (both routerboot and ROS) are lowest versions to which it's possible to downgrade.

Any particular reason to go with ROS 6.48? AFAIK there are no significant differences between 6.48 and 6.49 functionality-wise ... and latest 6.49 versions seem to be pretty stable (there are not many reports on problems with 6.49 on this forum).

Any particular reason to go with ROS 6.48? AFAIK there are no significant differences between 6.48 and 6.49 functionality-wise ... and latest 6.49 versions seem to be pretty stable (there are not many reports on problems with 6.49 on this forum).

6.48.2 can use 66960Mhz